CARBONIFEROUS PERIOD. 127 



much less, its lateral borders well defined, finely striated, botli vertically 

 and transversely ; fissure varying in proportional width, but usually quite 

 narrow, and completely closed by a pseudo-deltidium, which is more or less 

 flattened along each side, prominent along the middle, along which promi- 

 nence there is a slightly-raised mesial line. 



Dorsal valve capacious, more regularly convex than the other ; con- 

 vexity so great behind the middle as to carry a portion of the valve a little 

 behind the hinge-line, flattened a little toward the front, suggestive of a 

 mesial sinus, but seldom or never possessing a true one ; beak strongly 

 incurved, not projecting over the cardinal border ; area obsolete ; postero- 

 lateral poitions laterally compressed, leaving small thin ears at the hinge- 

 extremities. 



Surface of each valve marked by from ten to fourteen, more or less 

 angular, radiating plications, having deep, angular interspaces between them ; 

 plications not extending to the beak, increasing in size toward the front, 

 mostly simple, but sometimes bifurcating ; plications and interspaces both 

 marked by numerous fine radiating striae, which, toward the front margin of 

 adult shells, usually converge to the crests of the plications, upon which they 

 meet at acute angles ; crossing these converging lines, there are also usually 

 zig-zag lines of growth to be seen. The convergence of the radiating strise 

 does not take place until the shell has reached nearly mature size, and 

 occasionally not then. 



This shell is variable in size and shape, and to some extent in its surface- 

 markings also; but its general characteristics are siich as to separate it 

 widely from any associated forms. Two principal varieties of the species 

 have been recognized and published as separate species ; but in the Upper 

 Coal-Measiu-e rocks of Iowa I have found these two varieties associated 

 with such intennediate forms as to convince me that they are not specific- 

 ally distinct. Some of these difi^erences appear to be of such a character 

 only as all species are subject to, and some of them are evidently due to 

 difference in age alone. 



I have ■ not yet seen any American shell belonging to the genus Mee- 

 Icella that I regard as specifically distinct from M. striatocostata. Compared 

 with a specimen of Streptorhynchus pectimformis Davidson, sent me from 



