68 EOCENE MOLLUSCA. 



rounded ; pillar-lip with a large, lamelliform, elongated tooth, which appears to revolve 

 within the shell parallel to the suture ; a smaller raised line revolves nearer to the 

 columella within the shell, but becomes obsolete before it arrives at the pillar-lip. 

 Umbilicus large. Breadth 1 - 1 Oth of an inch." 



Taking the Texas shells, the form described by Say as the typical form of the 

 American species, the fossil shell presents, on comparison, the following variations : 



1st. The shell is somewhat smaller; the spire, except in the specimen I have 

 noticed, is more elevated, the apex not so obtuse, and the whorls are less convex. 



2d. The base of the shell is flatter, and the aperture not so rounded. 



3d. The position of the larger raised line is more median, and the smaller raised 

 line is wanting ; and, — 



4th. The peristome is simply reflected, and not " rounded" or thickened. 



Now it will be seen that the differences firstly mentioned are such as frequently 

 occur in a series of individuals of the same species. The variable height of the spire, 

 evidenced in the fossil shells by the depressed form of the specimen before mentioned, 

 is a character also found in the recent species ; since Gould, in his work above cited, 

 states expressly that " the shell varies considerably in the elevation of the spire, being 

 sometimes much flattened, and again it has a pointed apex ;" an observation, the 

 accuracy of which the Florida specimens in the British Museum fully confirm ; and 

 this difference in the elevation of the spire will depend on, and in fact will denote, the 

 less or greater convexity of the whorls. 



With respect to the flatter base, and the consequently less rounded aperture, the 

 same specimens from Florida exhibit a similar departure from the type ; in one instance, 

 indeed, the base is so much flattened as to impart a sub-carinated form to the basal 

 periphery of the whorl. 



The position of the larger tooth is equally variable in the Texas specimens ; and, 

 as regards the absence of the second or smaller raised line, Gould says that, " usually 

 but one of them (i. e. of the raised lines) exists ;" a statement, in fact, borne out by 

 some of the specimens from Ohio in the British Museum, in which the second line 

 is not perceptible. 



It is evident, then, that these variations, occurring as they do in the recent shells, 

 cannot afford sufficient grounds for a specific distinction of the fossil shell ; and the 

 only difference which apparently does not elude us on comparison, is the thickened or, 

 as Say describes it, the rounded outer lip of the recent shells. To rest specific 

 distinction on this character, one which, in general, is only an attribute of maturity, and 

 which, even if constant, could, at the utmost, merely serve to designate a variety, 

 would be an excess of refinement. But it cannot be affirmed that this variation is 

 constant, and a larger series of the fossil shells may show that even the thickened 

 outer lip is not wanting. Of the influence of varied conditions in modifying the form 

 of shells, very little is known or even conjectured ; but we may reasonably believe that 



