INVERTEBRATE PALAEONTOLOGY. 595 



moderately convex above, those of the spire being slightly covered above, 

 and almost entirely hidden below ; last turn a little depressed, with an 

 outward slope above, rather distinctly declining at the aperture, narrowly 

 rounded or (in the young shell) subangular around the periphery, flattened- 

 convex and rounding abruptly into the umbilicus below, where it is more 

 or less inflated toward the. aperture; umbilicus wide and deep; suture 

 distinct ; aperture transversely round-oval, being a little wider than high, 

 narrowly rounded on the outer side, and only very slightly modified by its 

 connection with the volution on the inner side; lip so much produced 

 above as to give a very oblique outline to its margin, which is nearly straight 

 above, but becomes more reflexed below, where it is a little constricted; 

 surface marked by moderately distinct, very oblique, lines of growth. 



• Greatest breadth, 1.82 inches; height about 1 inch; breadth of aper- 

 ture, 0.77 inch ; height, measuring vertically, about 0.50 inch, but measur- 

 ing obliquely from the advanced upper margin to its retreating lower edge, 

 about 0.88 inch. 



This is a noble species, that will be distinguished at a glance from the 

 other land-shells yet known from the Upper Missouri country, or, indeed, 

 from all of the Rocky Mountain region, by its large size, wide, deep umbil- 

 icus, and general form. It seems to present all of the characteristics of a 

 typical Macrocyclis, such as its depressed form, wide open umbilicus, the 

 ventricose character of its body-volution near the aperture below, and its 

 declining character at the same above, as well as its shortly reflexed lip on 

 the lower side and the constriction of its aperture there. 



It seems to have exceeded the typical M. laxata in size, and is readily 

 distinguished from it by its more prominent spire and its more narrowly- 

 rounded (or in the young) subangular periphery, caused by greater compres- 

 sion of its volutions above and below, particularly the latter. 



Locality and position. — From the Wind River Valley, where it was 

 found in the Wind River group ; probably of Eocene age. 



