Eusyringium japonicum Nakaseko, 1963, p.193, text-figs.20-21, pl.4, figs.1-3
Cyrtocapsella japonica (Nakaseko) Sanfilippo and Riedel, 1970, p.452, pl.1, figs.13-15 (with synonymy)
Shell consisting of three segments increasing uniformly in width, with a very constricted mouth. Cephalis spherical, poreless or with a few small circular pores, bearing a thornlike horn of the same length or shorter. Collar stricture not very distinct in contour because base of cephalis is generally enclosed in the thoracic wall. Thorax approximately hemispherical; abdomen swollen, rounded, with a mouth not much larger than a pore. Pores of thorax and abdomen generally similar, circular, usually closely spaced but occasionally sparser, rather regular in size and arrangement. Wall of thorax and abdomen thick, with rough surface. The lumbar stricture is not deep but is marked by a corresponding change in contour. Some specimens have a variable, inverted, caplike fourth segment with a thinner wall and less regular pores than in thorax and abdomen (Sanfilippo and Riedel, 1970).
Based on 30 specimens. Total length (excluding horn and fourth segment) 110-135 mm. Maximum breadth 55-100 mm (usually 75-90 mm). Ratio of length of abdomen to length of thorax (1.2-3.4):1, usually (1.8-2.4):1 (Sanfilippo and Riedel, 1970).
This species is distinguished from C. tetrapera by having the aperture of the third segment constricted, rather than that of the fourth. It is distinguished from C. elongata by having a more pronounced lumbar stricture, a rougher shell surface, and a larger skeleton (Sanfilippo and Riedel, 1970).
This species is cosmopolitan. Its morphotypic first and last appearances both lie within the Diartus petterssoni Zone.
For further taxonomic remarks see Sanfilippo and Riedel (1970).