CARBONIFEROUS SPECIES. 89 



the sinus of the other valve, and, as well as the lateral margins, costated in 

 the same way. 



Length, 1 inch; breadth, about 1.30 inches. 



I am not entirely satisfied that this shell is identical with -S^. opimus, 

 Hall ; though, so far as the specimens afford the means of comparison, it 

 seems to present no constant essential differences. Professor Hall's figure 

 in the Iowa report represents a short gibbous form of the shell with a 

 rather high area; but collections from the Coal-Measures of the Western 

 States show the shell to vary much in these characters. In fact, it is very 

 questionable whether there are any reliable specific differences between S. 

 opimus and S. Keokuk (including both Professor Hall's varieties of the latter) 

 and S. suhventricosuSj McChesney. Again, some varieties of these shells venj 

 nearly resemble certain forms of 8. increhescens, Hall ; while several of the 

 most distinguished European authorities consider the last-mentioned form 

 itself identical with S. Usulcatus, Sowerby, an opinion in which I am much 

 inclined to concur. It will also be noticed that our figure represents a shell 

 almost exactly agreeing with some varieties of S. hisulcatus, as may be seen 

 by comparing it with the variety of that species illustrated by fig. 8, 

 plate vii, of Mr. Davidson's Monograph of British Carboniferous Brachio- 

 poda. S. hisulcatus, seems to be, from the published illustrations, a variable 

 form; and, according to the limits assigned it by European authors, 

 might, so far as can be determined from comparison of external characters, 

 take in all of the proposed species and varieties mentioned above. Dr. 

 White considers S. opimus, HaW, and 8. JRocky-montana, Marcou, identical, 

 which may or may not be so ; but however this may be, there is about as . 

 good reason for viewing all of the shells here mentioned as varieties of -S'. 

 'hisulcatus, Sowerby. 



As I have not the necessary specimens at hand to attempt the solution 

 of the question in regard to the relations of these American forms to each 

 other and to 8. hisulcatus, I have merely referred our shell to 8. opimus. 

 Hall, provisionally, for the present. 



Locality and position. — Six miles south of Promontory Station, Promon- 

 tory Mountains, Railroad Canon, Moleen Peak; Mount Nebo, in Utah, 

 and at longitude 111° 38' W., latitude 40° 22' N. ; generally found in a 



