Homalopoma abeshinaiensis, Kaim & Jenkins & Hikida, 2009

Kaim, Andrzej, Jenkins, Robert G. & Hikida, Yoshinori, 2009, Gastropods from Late Cretaceous Omagari and Yasukawa hydrocarbon seep deposits in the Nakagawa area, Hokkaido, Japan, Acta Palaeontologica Polonica 54 (3), pp. 463-490 : 470-473

publication ID	https://doi.org/ 10.4202/app.2009.0042
persistent identifier	https://treatment.plazi.org/id/03FBF625-FFD4-2135-7D2E-939CFE542463
treatment provided by	Felipe
scientific name	Homalopoma abeshinaiensis
status	sp. nov.

Treatment

Homalopoma abeshinaiensis View in CoL sp. nov.

Fig. 4 View Fig .

Etymology: After the Abeshinai River in which the Omagari seep carbonate is located.

Holotype: UMUT MM30150 View Materials , Fig. 4C View Fig , moderately preserved shell with no protoconch.

Type locality: Omagari site, Nakagawa area , Northern Hokkaido, Japan. Coordinates 44 ° 39’26’’ N, 144 ° 2’25’’ E GoogleMaps .

Type horizon: Fossil hydrocarbon seep deposits of the Omagari Formation, Campanian, Upper Cretaceous.

Material.—269 moderately preserved specimens: 252 shells at UMUT and 17 shells at NMM.

Dimensions.—The holotype is 5.26 mm high and 4.57 mm wide.

Diagnosis.—Shell low turbiniform, whorls moderately inflated, ornamented by numerous spiral ribs and strongly prosocline collabral threads. Intersections of spiral and axial ornamentation nodose. Nodes elongated according to the direction of collabral threads. No clear demarcation between lateral flank and base of the shell. Base ornamented by numerous spiral ribs. H. abeshinaiensis differs from other species of Homalopoma in its clearly nodose intersections between spiral ribs and axial threads.

Description.—The protoconch is unknown. The shell is turbiniform with numerous spiral ribs. The earliest preserved whorls are ornamented by four spiral ribs but their number increases to eight on the later flank and eight on the base. The ribs are of similar strength although some secondaries are weaker. The ultimate whorl is characterized by a slightly dipping suture line. The generating curve is circular, aperture tangential, and peristome uninterrupted. The outer lip is smooth. The inner lip is usually with a narrow callus and there is no umbilicus but a shallow crescent−shaped depression between the inner lip and first spiral rib on the base.

Discussion.— Homalopoma abeshinaiensis recalls numerous Recent shallow−water Homalopoma −like collonins. However, the strongly ornamented species have usually no nodes on the spiral ribs [e.g., H. nocturnum (Gould, 1861) ] or the nodes occur only on primary ribs and are absent on the secondaries [e.g., H. amussitatum (Gould, 1861) ]. H. abeshinaiensis is very common at the Omagari seep but unknown so far from Yasukawa. H. abeshinaiensis is rather small as for this genus.

Stratigraphic and geographic range.—Omagari hydrocarbon seep in Nakagawa area of northern Hokkaido, Japan. Campanian, Upper Cretaceous.