verse, imbricating lann-ll.i-. seven or eight in a di-tanco of 5 mm. On the dorsal 

 of the shell these lamella: must be much farther apart. Revolving lines are 

 faintly indicated on the back. There is something so suggestive of Tremnnolus 

 tin- -[..-. !.- that it is to be hoped that better specimens may soon be 



Formation and toea/ity. Richmond froup of the Cincinnati period, near Spring Valley, Minnesota. 

 (MiMMM.-E. >. Cln 



1V.OSTOMA RICHMONDKN8IH, tl. Sp. (UMch.) 

 PLATE I.XVII. KI08. W and . 



Shell .-lightly exceeding medium size, the hight, including apertural expansion, 

 50 to 55 mm.; known from casts of the interior chiefly. These consist of about three 

 -tr.. n- volutions, the inner ones wide, depressed, sharply rounded on the aides, 

 broadly and evenly convex on the back, less convex and with a very slight central 



> on the ventral side, the whole giving a transversely elongate subelliptical 

 cross-section, whose width is a little more than twice the hight. I torso-ventral 

 diameter of last volution increasing very rapidly in the outer half, while the trans- 

 verse diameter enlarges very slowly. Just behind the apertural expansion, where 

 the volution is more or less distinctly compressed laterally, the dimensions in three 

 specimens (casts) are as follows: width 22, 23 and 24 mm.; hight 26, 27 and 27 mm. 

 At the opposite side of the shell the volution is about 15 mm. wide and 7 mm. high 

 in all three specimens. The umbilicus is of the usual size for the inner volutions, 

 I >ut for the entire shell it is comparatively small. This is because the angular or 

 narrowly rounded boundary moves gradually toward the ventral side of the 

 volutions, causing the wall of the umbilicus to become more and more abrupt. 

 lust behind the aperture it is nearly or quite perpendicular, the ventral surface of 

 the volution being almost flat. Apertural expansion abrupt, apparently not very 

 wiilo, with recurved edges, broadly ovate in outline, .slightly narrower above than 

 below. Dorsal slit about _'" in in. in length, beginning the same distance or some- 

 what more behind the apertural expansion. The slit is represented by a rough 

 (fractured) ridge on casts. Behind it the cast is smooth, but in front of it there is a 

 more or less distinct broad furrow. 



The surface markings have been observed only on the back of the second 

 volution. Here they consist of about seven irregular revolving ribs on each side of 

 a very narrow elevated slit-band. At intervals of about !.;" nun. the rilis are 

 interrupted by transverse lamella'. Where they are shown the volution has a 

 width of 7 mm. 



