16 Assembly 



this genus have the lower volutions free, and the apex consisting of one or 

 two minute contiguous volutions. From these forms we gradually pass to 

 otiiers wliere the apex is simply arcuate, and finally to tliose which are 

 entirely straight. From the large number of species grouped under this 

 designation, it might be doubted whether there is not room for farther sepa- 

 ration ; but with our present knowledge, it does not appear practicable to 

 draw any lines of generic distinction, and we find, moreover, that some of 

 the species, in their different stages of growth, present a variety of character 

 whicli might render it difficult to recognize them as identical under all cir- 

 cumstances. 



Although possessing numerous casts of some species of this genus, and a 

 large number of shells of several species from the interior of which the 

 stony matter is entirely separated, I have thus far been unable to recognize 

 the peculiar muscular impressions which are characteristic of Capulus or 

 PiLEOPSis ; and 1 feel therefore constrained, in the present state of my know- 

 ledge, to adopt the generic name Platyceras. 



Genus Platyceras ( Conrad, as emended ). 



Shells depressed subglobose, subovoid or obliquely subconical. Spire 

 small : volutions few, sometimes free and sometimes contiguous, with- 

 out columella ; aperture more or less expanded, often campanulate 

 and sometimes with the lip reflexed ; peristome entire or sinuous. 



Surface striated or cancellated, often spirally ridged or plicate, and 

 sometimes strongly lamellose transversely, nodose or spiniferous. 



Many of the species show a sinuosity of the striae, indicating a notch 

 iu the margin of the. aperture during the first stages of growth, and 

 this notch sometimes remains in the mature condition. More frequently, 

 however, the earlier sinus is closed, and, in certain species, the margin 

 continues unbroken, while in a few others this sinus is continued to the 

 margin at the final period of growth ; but more often it becomes closed 

 at some period during the growth of the shell, and another commenced 

 at some other point, and not seldom two or more are thus begun and 

 continued; while some simply striated species, with a single sinus in 

 their earlier stages of growth, become more or less plicated towards the 

 margin, with several sinuosities in the peristome in the mature condition : 

 usually, however, one or two of the marginal sinuosities are deeper than 

 the others.* 



* The variety of form here described, with extensive illustrations of more than 

 forty species in the third volume of the Palgeontology of New-York, convey 

 but an imperfect idea of the variet)'^ of character and expression presented in an 

 extensive collection of the species of this genus. Since the preceding observations 

 were written and printed (in Vol. in, p. 309), I have examined a collection obtained 

 at Cumberland, Md., in September 1858, and find some specimens which show the 



