NATURAL SCIENCES OP PHILADELPHIA. 273 



Length and breadth of a medium sized specimen, each 0-45 inch; length of 

 aperture, 0*25 inch; breadth of do., 0'23 inch ; apical angle convex, diverg- 

 ence, 90; breadth of spiral band at the aperture 007 inch. 



This species is nearly related to P. Beckwithana of McChesney (New Palaeo- 

 zoic fossils, p. 61), with which we supposed it to be identical from Prof. 

 McCbesney's description, until we had an opportunity to compare it with 

 good examples of the P. Beckwithana from the original locality. On com- 

 parison with these, we find our shell to be readily distinguished by having 

 its spiral band located midway between the middle and upper margin of the 

 body whorl, instead of passing around the middle of the outer side. It like- 

 wise differs in showing no traces of revolving stria? on the spiral band, and 

 in having small wrinkles crossing the revolving striae on the under side of the 

 body whorl, while the little wrinkles around the upper edge of the whorls 

 are stronger and shorter than in P. Beckwithana. Again there is a differ- 

 ence in the revolving striae, those of our shell never having an intermediate 

 smaller one between two larger ones, as is generally the case with those of 

 McCbesney's species. 



The close similarity between these two species, both in form and ornamen- 

 tation, shows the necessity for great care and precision in drawing up de- 

 scriptions of species, even where they may be widely different from all known 

 forms ; since we often find, in such cases, that other species are afterwards dis- 

 covered that cannot be distinguished by the original description from the 

 forms first described. Every word in Prof. McCbesney's description, except- 

 ing what is said in regard to the starting point of the spiral band, would ap- 

 ply equally well to our species. It is true, he gives the number of whorls as 

 four or five, while in our shell they may be described as numbering five or 

 six, but of course little reliance can be placed upon a difference of only one 

 whorl, where they are all counted to the extreme apex. 



Locality and position. Lower Coal Measures, on Hodge's Creek, Macoupen 

 County, Illinois. 



Pleurotomaria valvatiformis, M. & W. 



Shell minute, depressed, or about twice as wide as high ; volutions three 

 and a half to four, regularly rounded, and increasing rather gradually in size; 

 suture well defined in consequence of the convexity of the whorls ; umbilicus 

 proportionally small or closed ; aperture suborbicular, being a little straighter 

 on the inner side. Spiral band nearly or quite even with the surface of the 

 whorls, and placed on the middle of their outer side. Surface smooth, as 

 seen without a magnifier, but presenting traces of microscopic revolving striae, 

 in a good light under a strong lens. 



Height, 0-04 inch; breadth, 008 inch. 



This is by far the smallest species of the genus we have ever seen, and if it 

 were not for the fact that we find so many specimens of it not exceeding the 

 dimensions given above, we would think might be a young shell. This, how- 

 ever, taken in "connection with the absence, so far as yet known, of any spe- 

 cies in our carboniferous rocks agreeing near enough for this to be its young, 

 are sufficient reasons for believing it to be an adult shell. It is more nearly 

 like our P. micronema of this paper than any of its associates with which we 

 are acquainted, but in addition to its vastly smaller size (although having 

 nearly the same number of whorls), it differs in being much more depressed, 

 and in having proportionally much more slender whorls ; while its spiral 

 band passes around the middle of the body whorl, instead of between the 

 middle and the upper margin. In the position of its band it is nearer like 

 P. Beckwithana of McChesnej, but differs so widely in size, and other charac- 

 ters, as to render a close comparison unnecessary. 



Locality and position. Hodge's Creek, Macoupen County, 111. Lower Coal 

 Measures. 



1866.] 18 



