taxonID	type	description	language	source
F8378799B22FFF8B2CD0FCA3FC39FC09.taxon	vernacular_names	(New Japanese name: Nagamimi-hiramushi-zoku)	en	Tsuyuki, Aoi, Kohtsuka, Hisanori, Okuno, Junji (2025): Description of a new species of Prostheceraeus Schmarda, 1859 and redescription of a poorly known species, Phrikoceros sagamianus (Kato, 1937) comb. n., with inference of their phylogenetic positions within Pseudocerotoidea (Platyhelminthes: Polycladida: Cotylea). Journal of Natural History 59 (5 - 8): 303-330, DOI: 10.1080/00222933.2024.2439032, URL: http://dx.doi.org/10.6084/m9.figshare.28302561
F8378799B22FFF872C0EFC27FE4EF9FB.taxon	vernacular_names	(New Japanese name: Ichigo-nagamimi-hiramushi)	en	Tsuyuki, Aoi, Kohtsuka, Hisanori, Okuno, Junji (2025): Description of a new species of Prostheceraeus Schmarda, 1859 and redescription of a poorly known species, Phrikoceros sagamianus (Kato, 1937) comb. n., with inference of their phylogenetic positions within Pseudocerotoidea (Platyhelminthes: Polycladida: Cotylea). Journal of Natural History 59 (5 - 8): 303-330, DOI: 10.1080/00222933.2024.2439032, URL: http://dx.doi.org/10.6084/m9.figshare.28302561
F8378799B22FFF872C0EFC27FE4EF9FB.taxon	description	(Figures 1, 2)	en	Tsuyuki, Aoi, Kohtsuka, Hisanori, Okuno, Junji (2025): Description of a new species of Prostheceraeus Schmarda, 1859 and redescription of a poorly known species, Phrikoceros sagamianus (Kato, 1937) comb. n., with inference of their phylogenetic positions within Pseudocerotoidea (Platyhelminthes: Polycladida: Cotylea). Journal of Natural History 59 (5 - 8): 303-330, DOI: 10.1080/00222933.2024.2439032, URL: http://dx.doi.org/10.6084/m9.figshare.28302561
F8378799B22FFF872C0EFC27FE4EF9FB.taxon	description	ZooBank: urn: lsid: zoobank. org: act: E 0 C 83 AD 9 - 736 A- 4951 - B 4 E 8 - 5813 C 690 C 43 F	en	Tsuyuki, Aoi, Kohtsuka, Hisanori, Okuno, Junji (2025): Description of a new species of Prostheceraeus Schmarda, 1859 and redescription of a poorly known species, Phrikoceros sagamianus (Kato, 1937) comb. n., with inference of their phylogenetic positions within Pseudocerotoidea (Platyhelminthes: Polycladida: Cotylea). Journal of Natural History 59 (5 - 8): 303-330, DOI: 10.1080/00222933.2024.2439032, URL: http://dx.doi.org/10.6084/m9.figshare.28302561
F8378799B22FFF872C0EFC27FE4EF9FB.taxon	materials_examined	Material examined Holotype: NSMT-Pl 9505, sagittal sections of the whole body (13 slides), dredged from a depth of 122 – 179 m in Sagami Bay, off Misaki, Kanagawa, Japan (between 35.140117 ° N, 139.546833 ° E and 35.135533 ° N, 139.545783 ° E), on 10 June 2022. Paratypes: NSMT-Pl 9507, whole mount specimen, dredged from 101 – 105 m depth in Sagami Bay, off Misaki, Kanagawa, Japan (between 35.138555 ° N, 139.534663 ° E and 35.137662 ° N, 139.534970 ° E), on the same day as holotype; NSMT-Pl 9506, sagittal sections of the whole body (5 slides), the same collection data as NSMT-Pl 9507.	en	Tsuyuki, Aoi, Kohtsuka, Hisanori, Okuno, Junji (2025): Description of a new species of Prostheceraeus Schmarda, 1859 and redescription of a poorly known species, Phrikoceros sagamianus (Kato, 1937) comb. n., with inference of their phylogenetic positions within Pseudocerotoidea (Platyhelminthes: Polycladida: Cotylea). Journal of Natural History 59 (5 - 8): 303-330, DOI: 10.1080/00222933.2024.2439032, URL: http://dx.doi.org/10.6084/m9.figshare.28302561
F8378799B22FFF872C0EFC27FE4EF9FB.taxon	etymology	Etymology The specific epithet fragum, a Latin term signifying ‘ strawberry’, was chosen in reference to the dorsal colour pattern of the new species. This pattern is characterised by red maculae, resembling slices of a strawberry.	en	Tsuyuki, Aoi, Kohtsuka, Hisanori, Okuno, Junji (2025): Description of a new species of Prostheceraeus Schmarda, 1859 and redescription of a poorly known species, Phrikoceros sagamianus (Kato, 1937) comb. n., with inference of their phylogenetic positions within Pseudocerotoidea (Platyhelminthes: Polycladida: Cotylea). Journal of Natural History 59 (5 - 8): 303-330, DOI: 10.1080/00222933.2024.2439032, URL: http://dx.doi.org/10.6084/m9.figshare.28302561
F8378799B22FFF872C0EFC27FE4EF9FB.taxon	diagnosis	Diagnosis Pair of pseudotentacles slender and pointed, reddish purple in colour; translucent creamwhite background; red maculae oval in shape, only distributed radially along body periphery and over midline on dorsal surface; pair of cerebral-eyespot clusters, each composed of about 30 eyespots; marginal and frontal eyespots absent; pseudotentacular eyespots numerous; multiple uterine vesicles present in each uterus.	en	Tsuyuki, Aoi, Kohtsuka, Hisanori, Okuno, Junji (2025): Description of a new species of Prostheceraeus Schmarda, 1859 and redescription of a poorly known species, Phrikoceros sagamianus (Kato, 1937) comb. n., with inference of their phylogenetic positions within Pseudocerotoidea (Platyhelminthes: Polycladida: Cotylea). Journal of Natural History 59 (5 - 8): 303-330, DOI: 10.1080/00222933.2024.2439032, URL: http://dx.doi.org/10.6084/m9.figshare.28302561
F8378799B22FFF872C0EFC27FE4EF9FB.taxon	description	Description Body elongated oval, 11.1 – 12.5 mm in length (11 mm in holotype) and 3.9 – 4.8 mm at its widest point (3.9 mm in holotype) in living state (Figure 1 (A, B )), 7.9 mm in length and 4.3 mm at its widest point after fixation (Figure 1 (C )). Pair of pseudotentacles apparent, slender and pointed, coloured reddish purple, about 1 mm in length (Figure 1 (D )). Dorsal surface smooth, translucent cream with specific colour pattern; four dark-red maculae (0.52 – 0.90 mm in diameter in holotype) arranged in midline; around 30 – 50 oval red maculae (0.32 – 0.91 mm in diameter in holotype), distributed radially along body margin; basically larger one and smaller one arranged alternately (Figure 1 (A )); six triangular red maculae adjacent to each other distributed in anterior to posterior end of cerebral-eyespot clusters; red colouration fading towards centre in all maculae (Figure 1 (A )). Marginal lines absent. Colour pattern on dorsal surface generally fading away after fixation (Figure 1 (C )). Ventral surface translucent, reddish-purple pigments forming two dotted lines along body margin in specific area from anterior end of cerebral eyespots to level of 1 / 3 of pharynx; reddish-purple pigments sparsely distributed along body periphery posterior to dotted lines (Figure 1 (B )). Cerebral eyespots in two elongated clusters (0.3 – 0.4 mm in length), each composed of about 30 eyespots (Figure 1 (D )). Frontal and marginal eyespots absent. Pseudotentacular eyespots distributed at base of each tentacle, each cluster consisting of about 30 eyespots (Figure 1 (C, D )). Intestine highly branched, anastomosing (Figure 1 (B )). Pharynx plicated, tubular and bell-shaped, 0.82 mm in length in fixed specimen (Figure 1 (B, C )). Mouth opening at anterior end of pharyngeal pouch. Male and female gonopores opening separately behind posterior end of pharynx (0.46 mm and 0.86 mm behind, respectively) (Figure 1 (C )). Male copulatory apparatus with large seminal vesicle, spherical prostatic vesicle, and armed penis papilla (Figure 2 (A, B )). Pair of sperm ducts forming common spermiducal vesicle before entering proximal end of seminal vesicle (Figure 2 (A, B )). Seminal vesicle oval, 390 µm in long axis and 150 µm in short axis, coated with thick muscular wall, leading ejaculatory duct (Figure 2 (A )). Prostatic vesicle spherical, 136 µm in diameter, coated with thin muscular wall; inner wall of prostatic vesicle lined with glandular epithelium consisting of tall cells (ca. 50 µm in length) (Figure 2 (B )). Prostatic duct opening into penis papilla. Ejaculatory duct connecting to prostatic duct before proximal end of penis papilla (Figure 2 (A )). Penis papilla bearing cuticular stylet 100 µm in length, surrounded by penis sheath (Figure 2 (B )). Male atrium opening to exterior via male gonopore, 146 µm in diameter (Figure 2 (A, B )). Female gonopore located at distance of 0.6 mm posterior to male gonopore (Figures 1 (C), 2 (A )). Pair of uteri filled with multiple eggs, each having 3 – 4 uterine vesicles (Figure 2 (C )), running anteriorly on both sides from 1.4 mm distance from posterior end of body (Figure 1 (B, C )), fusing before entering proximal end of vagina (Figure 1 (C )). Lang’s vesicle absent. Vagina curving down, leading to cement pouch (Figure 2 (A, D )). Cement glands well developed around female copulatory apparatus, releasing their contents in cement pouch (Figure 2 (A, D )). Female atrium opening to exterior via female gonopore (Figure 2 (A )). Sucker 0.3 µm in diameter, situated at distance of 1.7 mm posterior to female gonopore (Figure 1 (C )).	en	Tsuyuki, Aoi, Kohtsuka, Hisanori, Okuno, Junji (2025): Description of a new species of Prostheceraeus Schmarda, 1859 and redescription of a poorly known species, Phrikoceros sagamianus (Kato, 1937) comb. n., with inference of their phylogenetic positions within Pseudocerotoidea (Platyhelminthes: Polycladida: Cotylea). Journal of Natural History 59 (5 - 8): 303-330, DOI: 10.1080/00222933.2024.2439032, URL: http://dx.doi.org/10.6084/m9.figshare.28302561
F8378799B22FFF872C0EFC27FE4EF9FB.taxon	distribution	Type locality and distribution	en	Tsuyuki, Aoi, Kohtsuka, Hisanori, Okuno, Junji (2025): Description of a new species of Prostheceraeus Schmarda, 1859 and redescription of a poorly known species, Phrikoceros sagamianus (Kato, 1937) comb. n., with inference of their phylogenetic positions within Pseudocerotoidea (Platyhelminthes: Polycladida: Cotylea). Journal of Natural History 59 (5 - 8): 303-330, DOI: 10.1080/00222933.2024.2439032, URL: http://dx.doi.org/10.6084/m9.figshare.28302561
F8378799B22FFF872C0EFC27FE4EF9FB.taxon	materials_examined	Specimens were collected from the sublittoral zone at a depth of 122 – 179 m in Sagami Bay, off the coast of Miura Peninsula, Kanagawa, Japan (between 35.140117 ° N, 139.546833 ° E and 35.135533 ° N, 139.545783 ° E), at 122 – 179 m depth. Tentatively identified euryleptid polyclads with a similar dorsal colour pattern have also been reported from the sublittoral zone in Indonesia (Ryanskiy 2021).	en	Tsuyuki, Aoi, Kohtsuka, Hisanori, Okuno, Junji (2025): Description of a new species of Prostheceraeus Schmarda, 1859 and redescription of a poorly known species, Phrikoceros sagamianus (Kato, 1937) comb. n., with inference of their phylogenetic positions within Pseudocerotoidea (Platyhelminthes: Polycladida: Cotylea). Journal of Natural History 59 (5 - 8): 303-330, DOI: 10.1080/00222933.2024.2439032, URL: http://dx.doi.org/10.6084/m9.figshare.28302561
F8378799B22FFF872C0EFC27FE4EF9FB.taxon	biology_ecology	Habitat The collected specimens were found inhabiting coarse sand and seashell substrates on the seafloor at depths ranging from 100 to 180 m by dredging. An underwater photograph of a closely related species captured by a scuba diver in shallower waters (Ryanskiy 2021) suggests the possibility of the species occurring in shallower habitats (see Discussion section).	en	Tsuyuki, Aoi, Kohtsuka, Hisanori, Okuno, Junji (2025): Description of a new species of Prostheceraeus Schmarda, 1859 and redescription of a poorly known species, Phrikoceros sagamianus (Kato, 1937) comb. n., with inference of their phylogenetic positions within Pseudocerotoidea (Platyhelminthes: Polycladida: Cotylea). Journal of Natural History 59 (5 - 8): 303-330, DOI: 10.1080/00222933.2024.2439032, URL: http://dx.doi.org/10.6084/m9.figshare.28302561
F8378799B22FFF872C0EFC27FE4EF9FB.taxon	materials_examined	COI sequence The determined COI sequence (LC 856622) from the holotype (NSMT-Pl 9505) was 715 bp in length. BLASTn of the sequence returned 84.01 % identity with published Pseudobiceros stellae sequences (GenBank accession MT 896282 – 896283) at maximum, followed by 83.93 % of Pseudobiceros damawan sequence (MN 690539) and 83.79 % of Prostheceraeus roseus (MZ 273078). The sequence of the holotype of Pr. fragum sp. n. did not show the most similarity to the published Prostheceraeus sequence, but this could be due to different rates in query cover among the compared sequences (41 – 42 % for Pseudobiceros spp.; 71 % for Pr. roseus).	en	Tsuyuki, Aoi, Kohtsuka, Hisanori, Okuno, Junji (2025): Description of a new species of Prostheceraeus Schmarda, 1859 and redescription of a poorly known species, Phrikoceros sagamianus (Kato, 1937) comb. n., with inference of their phylogenetic positions within Pseudocerotoidea (Platyhelminthes: Polycladida: Cotylea). Journal of Natural History 59 (5 - 8): 303-330, DOI: 10.1080/00222933.2024.2439032, URL: http://dx.doi.org/10.6084/m9.figshare.28302561
F8378799B22FFF872C0EFC27FE4EF9FB.taxon	discussion	Remarks Based on the presence of key morphological features, the collected specimens were assigned to the genus Prostheceraeus. These features include: (1) a pair of slender, pointed pseudotentacles, (2) an intestine with a network of anastomosing branches, and (3) a pair of uteri with multiple uterine vesicles (Faubel 1984; Prudhoe 1985). So far, Prostheceraeus species have been distinguished from each other based on external body colour patterns (Cuadrado et al. 2021; Soutullo et al. 2021). Prostheceraeus fragum sp. n. can be distinguished from its congeners, Prostheceraeus boucheti Prudhoe, 1989, Prostheceraeus fitae Soutullo et al., 2021, Pr. fuscolineatus, Prostheceraeus giesbrechtii Lang, 1884, Prostheceraeus meleagrinus (Kelaart, 1858), Prostheceraeus pseudolimax Lang, 1884, Prostheceraeus roseus Lang, 1884, and Prostheceraeus vittatus (Montagu, 1815), by the complete absence of longitudinal lines on its dorsal surface (Figure 1 A; Table 2). Our new species can further be distinguished from Prostheceraeus crozieri (Hyman, 1939) and Prostheceraeus zebra Hyman, 1955 a by lacking any transversal lines on its dorsal surface (Figure 1 A; Table 2). Prostheceraeus albocinctus Lang, 1884, Prostheceraeus crisotomum Cuadrado et al., 2021, Prostheceraeus maculosus (Verrill, 1892), Prostheceraeus moseleyi Lang, 1884, Prostheceraeus nigricornus Schmarda, 1859, and Prostheceraeus rubropunctatus Lang, 1884 possess spotted or maculated dorsal surfaces. These markings differ from those of Pr. fragum sp. n. in several aspects: colouration (red or reddish purple in Pr. fragum sp. n.; brown to black in other species) and distribution (along the body periphery and midline in Pr. fragum sp. n.; scattered across the dorsal surface in other species) (Figure 1 (A); Table 2). Additionally, Prostheceraeus floridanus Hyman, 1955 b and Prostheceraeus panamensis Woodworth, 1894 differ from Pr. fragum sp. n. by possessing an irregular meshwork on their dorsal surfaces. Furthermore, our new species lacks marginal lines, unlike Prostheceraeus flavomarginatus (Ehrenberg, 1831) and Prostheceraeus violaceus Delle Chiaje, 1822 (Table 2). Prostheceraeus fragum sp. n. has a somewhat similar colour pattern to Prostheceraeus argus (Quatrefages, 1845), both exhibiting strings of purple dots along the body margins. However, Pr. fragum sp. n. can be distinguished by the absence of (1) white dots scattered over the dorsal surface and (2) marginal eyespots extending posteriorly, both of which are present in Pr. argus (Table 2). While information on the colouration of Prostheceraeus anomalus Haswell, 1907 is lacking, a distinct anatomical feature separates these species. Prostheceraeus anomalus possesses a unique vesicle, the ‘ receptacula seminalis ’, that connects to the vagina rather than the oviducts or uteri (Haswell 1907, p. 482), a feature absent in Pr. fragum sp. n. In conclusion, our new species Pr. fragum sp. n. can be morphologically distinguished from the 22 possible congeners.	en	Tsuyuki, Aoi, Kohtsuka, Hisanori, Okuno, Junji (2025): Description of a new species of Prostheceraeus Schmarda, 1859 and redescription of a poorly known species, Phrikoceros sagamianus (Kato, 1937) comb. n., with inference of their phylogenetic positions within Pseudocerotoidea (Platyhelminthes: Polycladida: Cotylea). Journal of Natural History 59 (5 - 8): 303-330, DOI: 10.1080/00222933.2024.2439032, URL: http://dx.doi.org/10.6084/m9.figshare.28302561
F8378799B225FF812C8CFF2FFC35FE85.taxon	vernacular_names	(New Japanese name: Kazenoko-hiramushi-zoku)	en	Tsuyuki, Aoi, Kohtsuka, Hisanori, Okuno, Junji (2025): Description of a new species of Prostheceraeus Schmarda, 1859 and redescription of a poorly known species, Phrikoceros sagamianus (Kato, 1937) comb. n., with inference of their phylogenetic positions within Pseudocerotoidea (Platyhelminthes: Polycladida: Cotylea). Journal of Natural History 59 (5 - 8): 303-330, DOI: 10.1080/00222933.2024.2439032, URL: http://dx.doi.org/10.6084/m9.figshare.28302561
F8378799B225FF9D2C6FFED8FCB0F98E.taxon	vernacular_names	(New Japanese name: Kanoko-nisetsuno-hiramushi)	en	Tsuyuki, Aoi, Kohtsuka, Hisanori, Okuno, Junji (2025): Description of a new species of Prostheceraeus Schmarda, 1859 and redescription of a poorly known species, Phrikoceros sagamianus (Kato, 1937) comb. n., with inference of their phylogenetic positions within Pseudocerotoidea (Platyhelminthes: Polycladida: Cotylea). Journal of Natural History 59 (5 - 8): 303-330, DOI: 10.1080/00222933.2024.2439032, URL: http://dx.doi.org/10.6084/m9.figshare.28302561
F8378799B225FF9D2C6FFED8FCB0F98E.taxon	description	(Figures 3, 4)	en	Tsuyuki, Aoi, Kohtsuka, Hisanori, Okuno, Junji (2025): Description of a new species of Prostheceraeus Schmarda, 1859 and redescription of a poorly known species, Phrikoceros sagamianus (Kato, 1937) comb. n., with inference of their phylogenetic positions within Pseudocerotoidea (Platyhelminthes: Polycladida: Cotylea). Journal of Natural History 59 (5 - 8): 303-330, DOI: 10.1080/00222933.2024.2439032, URL: http://dx.doi.org/10.6084/m9.figshare.28302561
F8378799B225FF9D2C6FFED8FCB0F98E.taxon	materials_examined	Material examined ICHUM 8977, sagittal sections of the reproductive organs (9 slides) along with the remaining unsectioned body (preserved in 70 % ethanol), collected from underwater cages used for ascidian cultivation in Misaki, Kanagawa, Japan (35.157383 ° N, 139.611917 ° E), on 10 June 2022. ICHUM 8978, sagittal sections of the reproductive organs (10 slides) along with the remaining unsectioned body (preserved in 70 % ethanol), collection data same as above. ICHUM 8979, whole mount specimen, collection data same as above. CMNH-ZC 209, preserved in 70 % ethanol, collected in intertidal rockly reef at Okino-shima Islet, Tateyama, Chiba, Japan (34.990833 ° N, 139.822783 ° E), on 19 March 2014.	en	Tsuyuki, Aoi, Kohtsuka, Hisanori, Okuno, Junji (2025): Description of a new species of Prostheceraeus Schmarda, 1859 and redescription of a poorly known species, Phrikoceros sagamianus (Kato, 1937) comb. n., with inference of their phylogenetic positions within Pseudocerotoidea (Platyhelminthes: Polycladida: Cotylea). Journal of Natural History 59 (5 - 8): 303-330, DOI: 10.1080/00222933.2024.2439032, URL: http://dx.doi.org/10.6084/m9.figshare.28302561
F8378799B225FF9D2C6FFED8FCB0F98E.taxon	diagnosis	Diagnosis Pair of pseudotentacles small, ear-like, held erect; pale brown to light olive green background, conspicuously darker along midline with numerous white maculae of various sizes and shapes scattered over surface; cerebral-eyespot cluster horseshoe shaped; pseudotentacular eyespots present in dorsal and ventral sides; pharynx plicated with simple folds; pair of sperm ducts or spermiducal vesicles opening separately posterolateral to seminal vesicle; ejaculatory duct and prostatic duct entering base of penis papilla independently.	en	Tsuyuki, Aoi, Kohtsuka, Hisanori, Okuno, Junji (2025): Description of a new species of Prostheceraeus Schmarda, 1859 and redescription of a poorly known species, Phrikoceros sagamianus (Kato, 1937) comb. n., with inference of their phylogenetic positions within Pseudocerotoidea (Platyhelminthes: Polycladida: Cotylea). Journal of Natural History 59 (5 - 8): 303-330, DOI: 10.1080/00222933.2024.2439032, URL: http://dx.doi.org/10.6084/m9.figshare.28302561
F8378799B225FF9D2C6FFED8FCB0F98E.taxon	description	Description Body elongated oval, 27.8 – 30.2 mm in length and 12.4 – 13.1 mm at its widest point in living state (Figure 3 (A, B )). Pair of pseudotentacles small, ear-like, held erect (Figure 3 (A, C )). Dorsal surface smooth, pale brown to light olive green background, conspicuously darker along median line with numerous white maculae of various sizes and shapes scattered all over surface; white maculae forming transversal lines along body periphery. Body margin fringed by narrow black line (Figure 3 (A )). Tips of pseudotenctacles white without black line (Figure 3 (A )). Ventral surface translucent, with narrow black line (Figure 3 (B )). Cerebral-eyespot cluster horseshoe shaped, 0.35 mm in length, composed of about 55 eyespots, distributed in translucent area (Figure 3 (D )). Frontal and marginal eyespots absent. Pseudotentacular eyespots numerous, sparsely distributed on both dorsal and ventral sides, extending from base to tip of pseudotentacle at about 0.67 mm length and 0.22 mm width along margin (Figure 3 (C )). Mouth, male gonopore, and female gonopore, and sucker present along midline on ventral side. Pharynx, 3.8 – 4.9 mm in length, plicated with simple folds, located at anterior 1 / 3 of body (Figure 3 (B )). Mouth situated centrally in pharynx. Male gonopore, 220 µm in diameter, located immediately behind posterior end of pharyngeal pouch (Figure 3 (B )). Male copulatory apparatus composed of large seminal vesicle, prostatic vesicle, and armed penis papilla (Figure 4 (A )). Pair of spermiducal vesicles running anteriorly, opening separately and posterolateral to seminal vesicle (Figure 4 (A )). Seminal vesicle elliptical (long axis 738 µm, short axis 299 µm), coated with 29 µm thick muscular layer (Figure 4 (A – C )). Ejaculatory duct not coiled, entering penis papilla. Prostatic vesicle elongated oval, lying horizontally at base of penis papilla, lined with smooth granular epithelium, covered by muscular wall (Figure 4 (C, D )). Prostatic duct entering penis papilla at its base independent of ejaculatory duct (Figure 4 (A, C )). Penis papilla, 510 µm in length, armed with tubular, straight penial stylet (246 µm in length, 43 µm in width), enclosed in penis pouch, protruding into male atrium (Figure 4 (A, B )). Male atrium opening to exterior via male gonopore (Figure 4 (A – C )). Female gonopore, 230 µm in diameter, situated at 0.86 mm posterior to male gonopore. Female copulatory apparatus composed of cement pouch surrounded by numerous cement glands, vagina, and uteri. Vagina leading to female atrium across cement pouch (Figure 4 (E )). Female atrium opening to exterior via female gonopore. Cement glands well developed and concentrated around cement pouch (Figure 4 (E )). Sucker 0.44 – 0.49 mm in diameter, situated slightly anterior to middle of body (2.66 mm posterior to female gonopore) (Figures 3 (B), 4 (F )).	en	Tsuyuki, Aoi, Kohtsuka, Hisanori, Okuno, Junji (2025): Description of a new species of Prostheceraeus Schmarda, 1859 and redescription of a poorly known species, Phrikoceros sagamianus (Kato, 1937) comb. n., with inference of their phylogenetic positions within Pseudocerotoidea (Platyhelminthes: Polycladida: Cotylea). Journal of Natural History 59 (5 - 8): 303-330, DOI: 10.1080/00222933.2024.2439032, URL: http://dx.doi.org/10.6084/m9.figshare.28302561
F8378799B225FF9D2C6FFED8FCB0F98E.taxon	distribution	Type locality and distribution The species was initially described from ‘ Bentensita’ (likely equivalent to Bentenshima), Misaki, Kanagawa, Japan (Kato 1937). Subsequent records have documented its presence in Onagawa, Miyagi, approximately 400 km from the type locality (Kato 1939), and the present study reports its occurrence in Tateyama, Boso Peninsula, located across the mouth of Tokyo Bay.	en	Tsuyuki, Aoi, Kohtsuka, Hisanori, Okuno, Junji (2025): Description of a new species of Prostheceraeus Schmarda, 1859 and redescription of a poorly known species, Phrikoceros sagamianus (Kato, 1937) comb. n., with inference of their phylogenetic positions within Pseudocerotoidea (Platyhelminthes: Polycladida: Cotylea). Journal of Natural History 59 (5 - 8): 303-330, DOI: 10.1080/00222933.2024.2439032, URL: http://dx.doi.org/10.6084/m9.figshare.28302561
F8378799B225FF9D2C6FFED8FCB0F98E.taxon	biology_ecology	Habitat In Onagawa, previous studies reported the species from submerged slates (Kato 1939). The present study observed numerous specimens inhabiting underwater cages used for ascidian aquaculture.	en	Tsuyuki, Aoi, Kohtsuka, Hisanori, Okuno, Junji (2025): Description of a new species of Prostheceraeus Schmarda, 1859 and redescription of a poorly known species, Phrikoceros sagamianus (Kato, 1937) comb. n., with inference of their phylogenetic positions within Pseudocerotoidea (Platyhelminthes: Polycladida: Cotylea). Journal of Natural History 59 (5 - 8): 303-330, DOI: 10.1080/00222933.2024.2439032, URL: http://dx.doi.org/10.6084/m9.figshare.28302561
F8378799B225FF9D2C6FFED8FCB0F98E.taxon	discussion	Remarks Kato (1937) first described this species as Pseudoceros sagamianus based on a single specimen collected from Misaki, Kanagawa, Japan. The original description included a detailed account of the morphology and a black-and-white photograph of the live colouration (Kato 1937). Later, Kato (1939) reported an additional specimen of Ps. sagamianus from Konorihama, Onagawa Bay, Miyagi, Japan (Kato 1939). Unfortunately, during WWII, polyclad material studied by Kojiro Kato (1906 – 1981) was destroyed by the air raid on Tokyo in 1945 (Okuno et al. 2012). Consequently, the topotypic specimens examined herein are crucial for understanding the taxonomic status of Ps. sagamianus. Our specimens were assigned to Ps. sagamianus based on the following morphological characters: (1) a dorsal surface with white maculae of varying sizes and shapes distributed over a brown background, (2) a narrow black fringe bordering the body periphery, and (3) separate openings of the paired sperm ducts into the seminal vesicle. As noted by Kato (1939), Ps. sagamianus can be distinguished from the morphologically similar Ps. exoptatus Kato, 1938 b distributed in Seto, Wakayama, Japan, by the absence of a common sperm duct in Ps. sagamianus (fig. 22 in Kato 1937; fig. 16 in Kato 1938 b). The current taxonomic status of Phrikoceros Newman and Cannon, 1996 b is unclear, with some suggesting its synonymy with Tytthosoceros Newman and Cannon, 1996 a (see Bahia 2022). Both genera were previously distinguished from other pseudocerotid genera by the presence of (1) a slightly folded pharynx, (2) a single male copulatory organ, and (3) a single female copulatory organ (Newman and Cannon 1996 a, 1996 b). However, the differentiation based solely on pseudotentacle shape (double-folded or complex in Phrikoceros vs pointed or ear-like in Tytthosoceros; Bahia 2022) is unreliable due to the potential to misinterpret or overlook this feature. Consequently, Litvaitis et al. (2019) synonymised Tytthosoceros with Phrikoceros based on the phylogenetic placement of Tytthosoceros mopsus (Marcus, 1952) which had been misidentified as Phrikoceros mopsus (see Bahia 2022). In contrast, Bahia (2022) argued for the reinstatement of Tytthosoceros as a valid genus, highlighting the misidentification of T. mopsus. Resolving the taxonomic confusion between these genera requires further investigation, involving more molecular data especially from the type species of Phrikoceros, as suggested by Bahia (2022). Therefore, we adopt a conservative approach, considering both genera as Phrikoceros sensu lato (s. l.) and provisionally including all Tytthosoceros species herein. Nine species are currently recognised within Phrikoceros s. l.: Phrikoceros baibaiye Newman and Cannon 1996 b, Phrikoceros diadaleos Newman and Cannon, 1996 b, Phrikoceros fritillus Newman and Cannon, 1996 b, Phrikoceros galacticus Newman and Cannon, 1996 b, Phrikoceros katoi Newman and Cannon, 1996 b, Phrikoceros lizardensis (Newman and Cannon, 1996 a), and Phrikoceros nocturnus (Newman and Cannon, 1996 a) in the Indo-Pacific (Newman and Cannon 1996 a, 1996 b; Bolaños et al. 2016). Additionally, Phrikoceros inca (Baeza et al., 1997) is found on the Pacific coast of South America (Baeza et al. 1997; Bahia et al. 2012, 2014; Araya and Aliaga 2016; Bahia and Schrödl 2018; Reyes et al. 2020), and Ph. mopsus is found on the Atlantic coast of South America (Marcus 1952; Quiroga et al. 2004; Bulnes et al. 2011). In Japan, Ph. baibaiye, Ph. diadaleos, and Ph. fritillus, along with three additional unidentified species, have been reported solely from Okinawa based on photographic records (Ono 2015). We propose transferring Ps. sagamianus to Phrikoceros s. l. based on several morphological characters: the pharynx (plicated with shallow, simple folds in our specimens; Figure 3 (B )), body margin characteristics, and pseudotentacle morphology. Remarkably, the plicated pharynx of our specimens differs from the ‘ butterfly-shaped’ pharynx with complex pharyngeal lobes of Pseudoceros species (see Newman and Cannon 1994). However, limitations exist in definitively confirming this distinction due to the absence of type material and insufficient detail in the original descriptions by Kato (1937, 1939). The morphology of this species, particularly the deeply ruffled body margin of this species (unlike the smooth margins in Pseudoceros) and the erect pseudotentacles (in contrast to the ‘ folded flaps’ described for Ps. sagamianus by Kato 1939, p. 76), aligns better with Phrikoceros sensu lato (s. l.) than with Pseudoceros (Newman and Cannon, 1994, 1996 b). The species also corresponds to the diagnosis of Tytthosoceros in having (1) the ear-like shape of the pseudotentacles and (2) dorsal pseudotentacular eyespots not in four clusters, which are supposed to distinguish Tytthosoceros from Phrikoceros. Therefore, depending on the confirmation of the taxonomic separation between Phrikoceros and Tytthosoceros, a future transfer to Tytthosoceros might be warranted. Phrikoceros sagamianus comb. n. exhibits a colour pattern similar to Ph. mopsus, originally described from the Atlantic coast of Brazil (Marcus 1952; Quiroga et al. 2004; Bulnes et al. 2011; Bahia et al. 2012, 2014). The two species share a reddish-brown background with white blotches and a narrow black rim on the dorsal surface. However, Ph. sagamianus comb. n. can be distinguished by having white maculae forming transversal lines along the body periphery based on our specimens, contrasting with the absence of those maculae in Ph. mopsus (Bulnes et al. 2011; Bahia et al. 2012, 2014). In addition to the colour pattern, the species share internal reproductive features: (1) separate openings of the paired sperm ducts (or spermiducal vesicles) into the seminal vesicle, and (2) independent entry of the ejaculatory duct and the prostatic duct into the base of the penis papilla (Marcus 1952; Bulnes et al. 2011).	en	Tsuyuki, Aoi, Kohtsuka, Hisanori, Okuno, Junji (2025): Description of a new species of Prostheceraeus Schmarda, 1859 and redescription of a poorly known species, Phrikoceros sagamianus (Kato, 1937) comb. n., with inference of their phylogenetic positions within Pseudocerotoidea (Platyhelminthes: Polycladida: Cotylea). Journal of Natural History 59 (5 - 8): 303-330, DOI: 10.1080/00222933.2024.2439032, URL: http://dx.doi.org/10.6084/m9.figshare.28302561
