36 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I48 



2. Costae high, strong ; stolidium completely 



absent Septacamera 



B. Stenoscismatacea without stolidium Atriboniidae 



I. Atriboniidae with intercamarophorial plate . . . Atriboniinae 



1. Valves meeting nearly in plane at anterior . . . Atribonium* 



2. Numerous low costae, valves meeting at 



sharp angle Sedenticellula 



3. Costae very weak or absent, fold very high .... Camerisma 

 II. Atriboniidae without intercamarophorial 



plate Psilocamarinae 



1. Edges of camarophorium touching hinge plate 



a. Globular shape, weakly hemicostate Cyrolexis 



b. Numerous fine costae over most of 



shell Camarophorinella 



2. Smooth shell 



a. Uniplicate commissure Psilocamara 



b. Sulcate commissure Camarophorina 



* Small Coledium that lacks a stolidium can be distinguished from Atribonium 

 by the sharp angle of meeting of the valves at the anterior, and commonly by the 

 weak or absent costae. 



