GASTROPODA. 957 
Synopsis of Pleurotomartida.| 
Euconospira is most probably related to the Mesozoic group of species which 
Fischer has distinguished by the name Pyrgotrochus, with Pl. bitorquata Deslong- 
champs, as the type. The latter differs from our genus principally in having the 
band near the middle of the dorsal side of the whorls instead of at the basal 
edge. ‘The presence of a long slit distinguished the genus from Huconia, Hotomaria 
and Clathrospira. 
XII. Treposprra, n. gen. Shell sublenticular to depressed conical; base 
convex; without umbilicus; whorls, six or seven, the first two or three very small, 
rounded, prominent and smooth, the next two flat and coiled more or less nearly in 
the same plane, the rest sloping according to the apical angle of the shell; aperture 
transverse, subrhomboidal, the upper lip projecting beyond the lower, the outline 
curving strongly backward to the slit; edge of lower lip strongly convex in the 
middie; inner lip rather thin but continuing into a concave callosity which is spread 
over the umbilical region; slit very short, scarcely extending beyond the notch 
formed by the converging lips; band rather wide, slightly concave, smooth, visible 
on the last whorl only, so situated that its lower edge forms the peripheral angle 
of the volution; beginning with the fourth turn the sutural edge bears a row of 
nodes covering the band of the preceding whorl. The rest of the surface is nearly 
smooth, the lines of growth being nearly always obscure. Type, Pl. sphwrulata 
Conrad. 
In its most essential characters—form of shell, nearly smooth surface, short 
slit, and direction of lines of growth on the lower side—this genus resembles the 
Lower Silurian Liospira very closely. Still, we are fully satisfied that Trepospira is 
not a continuation of that early type. In coming to this conclusion we rely prin- 
cipally upon the character of the embryonic whorls. These, as stated, are rounded 
and smooth, wherefore we should look for the ancestors of the type among shells 
having similar whorls. The required conditions, it seems to us, are furnished by 
Pl. rotalia Hall, of the Hamilton group. In this species we see a callus filling the 
umbilicus, a nearly smooth surface, strongly curved lines of growth on the lower 
side of the whorls, the band hidden on the upper turns by overlap of the 
thickened and plicated sutural edge. Excepting the last, which is somewhat 
compressed and, therefore, obtusely angular, the whorls may properly be called 
rounded. Now, it is not a great step from Pl. rotalia to the small Upper 
Silurian shell which Lindstrém calls Pl. helicina in his great work on the 
Gothland Gastropoda, and more recently, because the name was preoccupied, 
Pl. kokent. The latter has neatly rounded whorls, an open umbilicus and 
the band a trifle too high, but in all other respects the agreement with PI. 
rotalia is sufficiently exact to indicate close genetic relations between them. As to 
