CARBONIFEROUS PERIOD. 145 



side, without a mesial fold, except that sometimes the IVont margin is slightly 

 raised to confoi'm to the shallow sinus of the other valve ; character of the 

 loop not fully determined, but it is knoAvn to reach farther forward than 

 the middle of the shell. 



Surface nearly smooth ; shell-structure finely punctate. 



This shell varies considerably in size and shape ; one specimen in my 

 private collection from Nebraska measuring thi'ee centimeters in length, 

 and is proportionally broad. Some in the collections from near Santa Fd 

 are unusually elongate, in which respect they seem to possess somewhat 

 definite varietal characters ; one of them measuring about twenty-five milli- 

 meters in length, thirteen millimeters in breadth, and ten millimeters in 

 height. The average size is about seventeen millimeters long and twelve 

 or thirteen millimeters broad. 



The presence of an elongated brachial loop in this shell, together with 

 the dental plates in the beak of the ventral valve, plainly shows that its 

 reference hitherto to the genus Terebratula proper is incorrect. Fragments of 

 the loop have been seen by breaking some of the solidly-filled shells of the 

 collections, but the best observations of that kind were made upon some 

 examples from Iowa. These were filled wdth calcite in the process of their 

 mineralization, the transparency of which allowed the loop to be seen by 

 transmitted light after the shell had been ground off, and polished above 

 and below. Only the general form and extent of the loop were ascertained, 

 as the details were obscured by the confused character of the crystalline 

 filling ; biit it is apparently much like that of Waldheimia. Besides the 

 dental plates, a broken example among the collections shows what is appar- 

 ently the bird-sternum-like process of the dorsal valve that characterizes 

 Bielasma, as described Ijy Professor King ; but this is not shown clearly. 

 The subgenus Dielasma is evidently closely allied to Gryptonella Hall ; and 

 if it is not really identical, the differences are apparently confined to internal 

 characters alone. Gryptonella is known to exist in the Subcarboniferous 

 strata of Michigan ; but its presence in strata of the Carboniferous period is 

 not certainly known. So far as indicated by the species I at present refer 

 to, Dielasma, the subgenus, seems to be confined to strata of the Carbonifer- 

 ous age, and to range through all three of its periods. Besides the species 



10 F 



