PELECYPODA—PRIONODESMACEA. 497 



*. Shell I inch high 365. P. texanus. 



3. Ribs strongly unequal, i. e., major and minor ribs d. 



d. Radially striate , 55. 



55. Ribs not prominent 374. P. [Chlamys) complexicosta. 



55. Ribs very prominent gg. 



gg. Major ribs unequal 386. P. (Chlamys) fucanus. 



gg. Major ribs equal 385. P. [Chlamys) eslrellanus. 



d. Not radially striate 66. 



66. Ribs not prominent 384. P. [Chlamys) marylandicus. 



66. Ribs very prominent hh. 



hh. Ribs bifurcating 387. P. [Chlamys) healeyi. 



hh. Ribs single 5f. 



5f . Anterior margin scalloped between the major ribs. ...**. 

 **. Shell very large, about 6 inches high. 



366. P. roemeri. 

 **. Shell less than 2 inches high. 



367. P. quinquecostatus. 

 5f. Anterior margin not scalloped. 



380. P. [Chlamys) perplanus. 



365. P. texanus Roemer. (Fig. 664.) Comanchic. 

 Surface of convex valve marked with 6 



rounded ribs the spaces between occupied by 

 two lower though equal, rounded ribs. 



Very abundant in the Washita, also present 

 in the Fredericksburg of Texas. 



366. P. (Vola) roemeri Hill. (Fig. 665.) 



Comanchic. 

 Large, thick, suboctahedral. Hinge line 

 two thirds length of shell. Both ears and -.„ Z 



& Fig. 664. Peiten lex- 



rest of shell marked with radiating costae, the anus. ( After Shattuck. ) 

 latter coarse and unequal. 



Very abundant in the upper Washita (Buda) of Texas ; also in 

 Mexico. 



367. P. (Neithea) quinquecostatus (Sowerby). (Fig. 666.) 



Comanchic-Cretacic. 



Right valve very convex, with incurved beak. Surface bearing 

 six strong, rounded, principal radiating ribs and two to four smaller 

 ones between ; ears also ribbed. Left valve flat. 



Comanchic : (Washita), Texas, northern Mexico. Cretacic : 

 New Jersey (Merchantville-Navesink). 



368. P. conradi (Whitfield). (Figs. 667-668.) Cretacic. 

 Surface of right valve crossed by crowded concentric folds 



