370 UNITED STATES GEOLOGICAL SURVEY OF THE TERRITORIES. 



than half the length of the aperture exclusive of the canal ; volutions five, 

 increasing rather rapidly in size from the apex, all obliquely flattened above; 

 last one vcntricose, but vertically flattened around the middle, where it is 

 ornamented by three more or less prominent, revolving, nodose or subnodose 

 carinas, only the upper one of which is seen on the spire ; canal long, tapering, 

 and slightly bent below; suture well defined; surface marked by distinct 

 lines of growth and raised revolving lines, one of which latter, at the middle 

 of each of the spaces between the three principal carinse of the body-volution, 

 as well as some of those below the same, show a tendency to assume the 

 character of small revolving ridges; aperture oval, but obtusely subangular 

 above, and abruptly contracting into the- rather narrow canal below; inner 

 lip thin, and spread upon the body-volution and upper part of the columella, 

 from which latter it seems to be raised to form the prominent fold or revolving 

 ridge of the same, nearly opposite the base of the aperture; while below this, 

 it stands free from the revolving umbilical ridge, so as to leave uncovered a 

 more or less distinct umbilical furrow, which is continued upward as a very 

 small perforation of the axis. 



Length of a medium-sized specimen, 3.50 inches; breadth, 2.15 inches; 

 length of aperture, exclusive of the canal, 1.34 inches; breadth of same, about 

 1.10 inches; length of canal, 1.70 inches. Imperfect specimens show that it 

 sometimes attained a size nearly one-third larger than these dimensions. 



This is, perhaps, one of the largest of known Cretaceous gasteropods, and 

 will be at once distinguished from all of those yet described from the Upper 

 Missouri rocks. How nearly it may be related to two forms described by 

 Professor Tuomey from the Cretaceous rocks of Noxubee County, Mississippi, 

 many years back, under the names Pyrula trochiformis and P. Richardsoni, I 

 am unable to say, not having seen authentic specimens of either; while Pro- 

 fessor Tuomey did not figure his species, and only gave very brief diagnoses, 

 doubtless intended as mere preliminary notices of shells that he expected to 

 describe more fully at some future time. I infer, however, that his species 

 are distinct from ours, because Mr. Gabb, who has studied the Mississippi 

 Cretaceous fossils, referred Professor Tuomey's P. Richardsoni to his Peris- 

 solax, which has a perfectly straight canal, without any prominence on the 

 columella; while lie expresses the opinion that P. trochiformis of Tuomey 

 may be identical with P.perlata, Conrad, the type of the genus; Pyropsis.* 



* Since this was written, Mr. Gabb informs me that he has seeu specimens of Pyrula Richardsoni, 

 Tuomey, very similar to the species here under consideration. 



