958 THE PALEONTOLOGY OF MINNESOTA. 
{Synopsis of Pleurotomariidaz. 
the umbilicus, this is not of much consequence, especially since Pl. kokeni has a 
ridge-like thickening of the shell around the umbilicus which may well represent 
the callus found in Pl. rotalia and typical species of Trepospira. From what line 
Pl. kokeni was derived, we are not prepared to decide, though inclined to regard 
Liospira as the most likely. Should that prove to be true, then Trepospira would 
furnish us an interesting case of reversion. Similar reversions are shown or noticed 
in other parts of this work. 
Descendants of the T’repospira type are to be looked for among the Mesozoic 
pleurotomarian shells which are commonly referred to Deslongchamps’ Cryptenia. 
The Triassic Pl. radians Wissman certainly resembles T’. depressa very closely, but 
Koken’s figure and description prove it to have a more inflated (“pear-shaped’’) 
embryonic whorl, and, if we understand him correctly, it has, like Pl. heliciformis, 
the type of Cryptenia, a narrow and long slit, which would exclude it from 
Trepospira. According to the figures which we have seen, Pl. polita Goldfuss 
(Lias), which also is usually referred to Cryptenia, agrees better with Trepospira 
so far as the position and width of the band and depth of the slit is concerned, 
but in the rounded form of its volutions, and, more importantly, in the direction 
of the lines of growth between the band and the umbilical callus, it differs 
widely. 
Besides the type, we place here the closely related Pl. depressa Cox, PI. 
illinoisensis Worthen, Raphistoma junior De Koninck, and possibly R. radians of the 
same author, all Carboniferous species. Perhaps it would be well to include the 
Hamilton Pl. rotalia Hall. 
XIII. Sretya, n. gen. (Ulrich.) Shell turbinate, consisting of from five to seven 
rapidly enlarging rounded or ventricose whorls, coarsely though usually not very 
deeply grooved spirally; apertural sinus shallow, broadly >shaped, slit wanting, 
band distinct, rather wide, more or less prominent, concave, nearly central on 
the last volution, infra-median on the upper turns; axis minutely perforated, 
the inner lip usually thin and reflected so as to form a hollow columella ; aperture 
slightly produced below. Type, S. ventricosa, n. sp. (Ulrich.) 
XIV. Prernosprra, n. gen. (Ulrich.) Rather short, turbinate shells, consisting 
of four or five rapidly enlarging ventricose whorls, the last produced below and 
greatly exceeding in hight the rest of the spire; band wide, submedian in position on 
the last whorl, flat or slightly concave, vertical, margined on each side by a raised 
line; apertural sinus not very deep, slit wanting; surface marked with lines and 
wrinkles of growth only; these are only moderately arched, especially upon the 
lower half of the volutions where, excepting near the band, they are nearly vertical. 
Type, Holopea cassina Whitfield. Range, Lower Silurian. 
