Genus Heteromysis S.I. Smith, 1873

Features shared by the three here described Heteromysis species. Carapace normal, without apparent sexual dimorphism (Fig. 9D). Rostrum represents a distinct horizontal, triangular plate. Cervical sulcus well developed; no cardial sulcus visible. Posterior margin of carapace evenly rounded, mid-caudally weakly emarginated.

Antenna. Presence of previously not described crescent-shaped shield proximally behind antennal peduncle on ventral face of antennal sympod. Size and shape of crescents may differ between species, e.g. H. korntalensis sp. nov. (Fig. 6I) versus H. schoenbrunnensis sp. nov. (dashed line in Fig. 9L).

Mouthparts and foregut. Labrum (Fig. 9M) caudally with small, stiff bristles. Fields of setae on caudal and ventral faces. Labium (Fig. 9N) normal, comprising two hairy lobes with short, dense set of stiff bristles on distal half of mesial face. Maxilla (Fig. 10E) normal, densely setose, with various types of setae, but no spines or teeth. Its leaf-like exopod not extending beyond basal segment of endopod. This segment plus the sympod, and all three large endites of the sympod, with densely setose distal margins. Mesial margin of sympod with a long seta bearing stiff bristles along its distal third; this seta extending beyond the dense brush of plumose setae. Gross structure of the foregut (Fig. 13A–E) as given in the first description of H. abednavandii Wittmann, 2020 .

Thoracic sternites. Sternite 1 anteriorly produced into a large lobe in both sexes (Fig. 10F versus 10I) contributing to the caudal closure of the mouth field as usual in Mysidae . Pair of barbed setae on intersegmental joint between thoracic sternite 2 and sympod 2. These setae show characteristic differences in size, thickness, and arrangement of barbs among each other (Fig. 10F). No such setae on sternites 1, 3–8.

Maxillipeds (thoracic endopods 1, 2). Coxa of maxilliped 1 (Fig. 10F) with small endite bearing one barbed seta at its tip. Basis with large, prominent endite that is densely setose on mesial margin. Ischium and merus each with one smaller but distinct, medially setose endite. Basis of maxilliped 2 with large, medially projecting endite (Fig. 10J).

Marsupium. Female thoracopods 7, 8 bear large oostegites 1 and 2, respectively. Each oostegite without setae on upper (dorsal) margin. Ventral margin and part of posterior margin, from subbasal region up to rounded tip, with dense series of plumose setae, together with the bilaterally opposite oostegite forming a gate contributing to the ventral and caudal closure of the marsupium. Marsupium inside with comparatively long setae microserrated on their distal half. Outer face of marsupium with short whip setae. Thoracopod 6 with small, terminally rounded, rudimentary oostegite (Fig. 11F) not contributing to wall of brood pouch. This rudiment may have some function by reaching into the brood pouch with a few microserrated setae.

Uropods. Exopod stout, its mesial margin more strongly convex than lateral margin (Fig. 4J). Mineral composition of statoliths is fluorite as also in 18 other Heteromysis species examined by Wittmann & Ariani (2019).