"I-ODA. 



of 



n>i>im i- ino-t probably related to the Mesozoic group of species which 

 ha- distinguished by the name /' '/"-. with /'/. Intonjuitta Deslong- 



champs, as the type. The latter differs from our genus principally in having the 

 band ii>ar the middle of the dorsal side of the whorl- instead of at the basal 

 edge. The presence of a long slit distinguished the genus from Euconia, Eotomaria 

 and Clathriispirii. 



XII. TRKI-OSHKA, n. gen. Shell sublenticular to depressed conical; base 

 convex; without umbilicus; whorls, six or seven, the first two or three very small, 

 rounded. |irominent au-1 -month. the next two flat and coiled more or less nearly in 

 tin- same plane, the rest sloping according to the apical angle of the shell; aperture 



-verse, subrhomboidal, the upper lip projecting beyond the lower, the outline 

 rurving strongly backward to the slit; edge of lower lip strongly convex in the 

 middle; inner lip rather thin but cunt inning into a concave callosity which is spread 

 the umbilical region; slit very short, scarcely extending beyond the notch 

 formed by the converging lip-: band rather wide, slightly concave, smooth, visible 

 on tin- la-t svhurl only, so situated that its lower edge forms the peripheral angle 

 of the volution: l.epnnin^ with the fourth turn the sutural edge bears a row of 

 nodes covering the ban<I of the preceding whorl. The rest of the surface is nearly 

 smooth, the lines of growth bein^ nearly always obscure. Type, /'/. sphttnilntn 

 t'onrad. F*. /wuls XXu* frJLr*~ S/*&J ^f&J****** 4** Off*d">f fapt 



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 Liof>ir<i very closely. Still, we are fully satisfied that Trepospirn 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 requireM conditions, it seems to us, are furnished by 

 /'/. rotnli'i II. ill. ot the Hamilton group. In this species we see a callus filling the 

 umbilicus, a nearly smooth 8Ui rongly curved lines of growth on the lower 



side of the whorl-. the band hidden on the upper turns by overlap of the 

 thickened and plicated -ntnral edge. Kxccpting the last, which is somewhat 

 compressed and. tl ohtu-ely angular, the whorls may properly be called 



rounded. Now, it is not a great step from /'/. /<,/,///,/ to the small Upper 

 Silurian shell which Lind-trom call.- /'/. Mirinn in his great work on the 

 <Mithland Gastropoda, and more recently, because the name was preoccupied, 

 /'/. /./.' /n. The latter has neatly rounded whorls, an open umbilicus and 

 the band a trifle too high, bat in all other respects the agreement with /'/. 

 rotalia is sufficiently exact to indicate close genetic relations between them. As to 



