LAMELLIBRANCHIATA. 307 



MICRODON, Conrad. 1842. 

 CYPRICARDELLA, Hall. 1858. 



EODON, Hall. 1877. 

 MICRODONELLA, (Ehleet. 1881. 



MiCRODON (CyPRICARDELLA) MAJOR, n. Sp. 

 PLATE XLII, FIG. 21. 



Shell large, rhomboid-ovate ; length one-third greater than the height ; basal 

 margin regularly curving ; more abruptly recurving at the anterior end and 

 less abruptly at the posterior ; posterior margin sub-truncate, slightly curv- 

 ing ; cardinal margin oblique, nearly straight. Anterior end broadly rounded. 



Valves moderately convex, not gibbous. 



Beaks, at about the anterior fourth of the shell, appressed. Umbonal 

 region distinctly convex, declining in an undefined ridge to the post-basal 

 cangle. 



Test comparatively thick, marked by strong, regularly rounded concentric 

 striae, which are abruptly recurved on the post-umbonal slope. Interior 

 unknown. 



The specimen described has a length of 73 mm., and a height of 50 mm. 



As compared with M. bellastriata it is a much larger form, with broader ante- 

 rior end, more extended at the post-basal extremity and the striae more abruptly 

 recurving over the umbonal slope. In the extension of the post-basal extrem- 

 ity it resembles M. complanata, but in other respects it is quite different. 



Formation and locality. In the Corniferous limestone of the Upper Helderberg 

 group, at Delaware, 0. 



