PELECYPODA—PRIONODESMACEA. 



483 



Large. Beaks erect, incurved. 



Mississippi : Nevada. Carbonic : Pennsylvania-Colorado. 

 324. S. curtus Meek and Worthen. Carbonic and Permic. 



FlG. 644. Schizodus cuneatus. (Ohio Pal., II.) 



Small, not exceeding \ inch in length, nearly circular, with 

 very elevated and nearly central beaks. 



Coal Measures : Indiana-Missouri. Permic : Kansas (Marion). 



FlG. 645. Schizodus wheeleri. 



b, internal molds 

 (Ind. Surv.) 



exterior of smaller left valve 



325. S. wheeleri Swallow. (Fig. 645.) Carbonic-Permic. 



Differs from S. chemungensis in outline, especially in the ob- 

 liquely truncated posterior portion. 



Coal Measures : Pennsylvania-New Mexico. Permic : Kansas, 

 Wyoming ? 



XC. Trigonia Bruguiere. 



Thick, trigonal, very inequilateral, with rounded anterior and 

 produced and angular posterior margin. Generally marked with 

 a ridge extending from the umbos to the posterior border, cutting 

 off the posterior dorsal area which has a different ornamentation. 



