PELECYPODA—ANOMALODESMACEA. 523 



442. P. subcompressa Meek. (Fig. 704.) Jurassic. 



Surface crossed by concentric ridges. Posterior end sloping. 



Widely distributed in the Jurassic from the plains to the Pacific 

 coast. 



Fig. 704. Pleuromya subcompressa, right and cardinal views. (After Logan.) 



443. P. inconstans Castillo and Aguilera. (Fig. 705.) Jurassic. 



A broad, shallow sinus usually present in the anterior umbonal 

 region. Posterior end high. 



Texas, Mexico. One of the most widely distributed fossils of 

 the Malone formation. 



Fig. 705. Pleuromya inconstans. (After Cragin.) 



CXX. Allorisma King. 



Equivalve, inequilateral, elongate, thin, arcuate. Anterior side 

 short ; posterior long and gaping. Beaks anterior, depressed. 

 Sculpture concentric, strongest medially. Hinge without teeth. 

 Ligament external. Cardinal margin inflected, forming a lanceo- 

 late depression along its border behind the beaks. Posterior ad- 

 ductor scar large. Pallial line sinuate. Carbonic and Permic. 

 444. A. geinitzi Meek. Carbonic. 



Small. Umbonal slopes carinated from the beaks posteriorly. 

 Surface marked with fine, radiating and concentric lines. 



Coal Measures : Illinois, Kansas. 



