958 THE PALEONTOLOGY OF MINNESOTA. 



[Synopsis of Pleurotomarildee. 



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 PI. 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 Trepospira type are to be looked for among the Mesozoic 

 pleurotomarian shells which are commonly referred to Deslongchamps' Cryptcenia. 

 The Triassic PI. 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 Cryptcenia, 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 Cryptasnia, 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, PL 

 illinoisensis Worthen, Eaphistoma junior De Koninck, and possibly R. radians of the 

 same author, all Carboniferous species. Perhaps it would be well to include the 

 Hamilton PI. rotalia Hall. 



XIII. SEELTA, 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. PLETHOSPIRA, 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. 



