PELECYPODA—PRIONODESMA CEA . 



393 



55. B. retrostriata von Buch. (Fig. 501.) Devonic. 



Shell very convex. Umbos large, closely incurved. Surface of 

 ribs flat or concave with backward curving growth lines ; grooves 

 narrower than ribs. 



Portage : New York (also Genesee), Pennsylvania, Appa- 

 lachians. 



XXV. Ctenodonta Salter. 



Equivalve. Surface smooth or with concentric growth lines. 

 Hinge arcuate, with a series of curved, transverse teeth which is 

 at times interrupted beneath the beaks. Cardinal area not 

 striated. Ligament small, immediately behind the beaks. Ad- 

 ductor muscle impressions nearly equal. Pallial line simple. 



Differs from Nuciila in the absence of resilifer beneath the beak, 

 and in the absence of a flattened area (lunule and escutcheon). 

 Ordovicic and Siluric. 



Ordovicic. 



Fig. 502. a, b y Ctenodonta logani ; c, d, C. gibberula y exterior of right valve and 

 internal mold of left ; e,f, C. socialis (/= hinge of right valves enlarged, X 3) '■> g> ^> 

 C.fecunda, right and cardinal views ; i t J, C. obliqua, internal molds, X 5 '■> &> ^ C- 

 alta ; m, C. albertina ; n y C. ca/vini, internal mold of left valve ; o, C. nasuta, left 

 valve. (Minn. Surv.) 



Teeth in a continuous and almost straight series. 

 Stones River : Tennessee, Illinois, Wisconsin. Black River : 

 New Jersey, Tennessee, Illinois, Minnesota, Wisconsin, Ontario. 



57. C. gibberula Salter. (Fig. 502, c-d.) Ordovicic. 



Hinge strongly bent. Muscle scars prominent. 



Stones River : Minnesota. Black River : Tennessee, Illinois, 

 Wisconsin, Iowa, Ontario. 



