CAEBONIFEEOUS PERIOD. 119 



some specimens they have been removed so completely that they appear as 

 if they had borne none. 



Dorsal valve concave, having a faint mesial fold in front corresponding 

 with the sinus of the other valve. No spines have been observed on the 

 dorsal valve; but in other respects its surface is marked like that of the 

 other. 



Length of a large specimen from Meadow Creek, Utah, eighteen milli- 

 meters; length of a specimen of a variety obtained from near Santa F^, 

 New Mexico, twelve millimeters; breadth of the one last mentioned, thir- 

 teen millimeters. 



The fact that Sowerby, who first described this species, divided it up 

 into four, and that Phillips and Keyserling each added another one, is suf- 

 ficient to indicate its variable character in Europe. The American forms 

 are now almost imiversally regarded as identical with the Em'opean, and it 

 is an interesting and significant fact that the species is as variable in this 

 part of the world as its European representative is. Some of the American 

 varieties are quite constant. The collections contain several fine examples, 

 of one of these varieties from near Santa F^, New Mexico, which variety is 

 as worthy of a separate specific designation as any of those that have been 

 mentioned. This variety is represented by figures 5 c and d, Plate VIII. 

 It uniformly smaller than the average size of typical examples ; the mesial 

 sinus is obsolete, spines delicate, and the surface upon the visceral region 

 more or less distinctly wrinkled. 



Taking the view of the identity of this species that is indicated by the 

 foregoing remarks, and the synonymy herewith presented, its geographical 

 and geological range is found to be very great. It is a well-known fossil in 

 the Carboniferous rocks of Europe, and also those of both North and South 

 America. In this country, it is found to range through the whole series of 

 strata of the Carboniferous period; but, so far as I am aware, it has not thus 

 far been found in the Subcarboniferous rocks of North America. 



Position and locality. — Strata of the Carboniferous period; east of 

 Minersville, and at Meadow Creek, south of Fillmore, Utah; Camp Cot- 

 tonwood, old Mormon road, Lincoln County, Nevada; near Santa F(^, 

 New Mexico. 



