DESCRIPTIONS OF SPECIES. 301 



developed posteriorly and supports the hinge plate. Its anterior extremity divides 

 the median pair of muscles. The inner surface of the hinge plate of Dielasina is 

 often marked by ■ transverse flutings, and this structure has, therefore, naturally 

 been interpreted as a surface of muscular attachment, beneath which it has some- 

 times been considered the viscera were lodged. The analogy between Oransena 

 and Dielasma is so strong throughout that the conclusion seems warranted that a 

 pair of muscles, leaving scars too faint to be often preserved, was attached to the 

 bottom of the shell anterior to the hinge plate in Dielasma, just as in Cransena, 

 and that the hinge plate in Cransena, which is also fluted, bore other muscles, as 

 is supposed to be the case in Dielasma. 



Hall and Clarke" correctly describe the hinge plate of these dielasmatoid forms, 

 but their figures are misleading in that the most enlarged and elaborate ones fail 

 to show this structure at all, while in the smaller ones it is inconspicuous and 

 somewhat slurred, over. For example, in the figure of Eunella sullivanti, the type 

 species of Eunella (fig. 23 of pi. 80), the crura spring almost immediately from the 

 posterior wall of the valve, and one would infer that the hinge plate was either 

 absent or an inconsiderable structure. The same feature is shown in Eunella 

 lincMseni (fig. 30 of same plate), where nothing resembling the cardinal plate 

 seems to be present. That a similar condition obtained in Cransena one would be 

 led to infer from the figure of the type species. Or. romingeri (fig. 16 on same 

 plate). 



I have referred to Cransena, Cransena suhelliptica, which Hall and Clarke 

 described as a Cryptonella, because it seems to have the characteristic brachidium 

 of. Cransena and because it seems to be so closely allied to Cransena ioioensis. 

 It resembles the latter not only in the general physiognomy of the shell, but in 

 the structure of the hinge plate and the shape, strength, and arrangement of the 

 muscle scars of the dorsal valve. In fact, there seems to be the closest agree- 

 ment throughout between these two forms. For my own part, 1 incline to lay 

 greater stress upon the structure of the hinge plate than upon minor crural char- 

 acters and to rank Cransena with Eunella as a subgenus of Cnjptonella, rather 

 than allow it to occupy that position with regard to Dielasma. 



The specimens from Colorado, while often abundant, are so imperfect as to 

 preclude a satisfactory determination. The generic characters, so far as I have 

 been able to ascertaiii them, agree with those of Cransena iowensis and Cr. svh- 

 elliptica. I feel considerable assurance that the shell is not a Dielasma. For all 

 1 am able to determine, however, it might be a representative of either Crypto- 

 nella, Cransena, or Eunella. In specific character it approaches Cr. svhelliptica. 

 Some examples have the same broadlj-^ oval form which characterizes that species, 



a Pal. New York, vol. 8, pt. 2, 1894, p. 297. 



