INSTITUTE OF SCIENCE, PHILADELPHIA. 53! 



FAMILY ANATINID/E. 



Soft parts like Pholadomya, the foot small and grooved, the ventral fora- 

 mina small, the siphons with a horny integument, not entirely retractile. 



Shell subequivalve, truncate or gaping behind, edentulous, the resilium 

 internal between two spoon-like chondrophores, vertically directed and often 

 supported by buttresses ; the ligament obsolete or absent ; area obsolete ; the 

 beaks transversely fissured, pallial sinus well marked ; moniecious, marine. 



Jurassic to the recent fauna. 

 Ex. Anatina, Periplomya, Cercomya, Rhynclwmyci, Anatiinya, Plalymya. 



FAMILY PERIPLOMIDyE. 



Soft parts generally like Anatina, but the siphons separated to their 

 bases, naked and wholly retractile, but not invertible. 



Shell subnacreous, conspicuously inequivalve, nearly closed, edentulous; 

 the resilium internal, between two anteriorly or vertically directed chondro- 

 phores, often buttressed, the lithodesma rarely wanting ; ligament and area 

 absent ; the beaks fissured, the pallial sinus broad and shallow ; monoecious, 

 marine. 



Tertiary and recent faunas. 

 Ex. Periploma, Cochlodesma, Bonttca, ? Tyleria. 



This group has erroneously been described as having the siphons united. 



FAMILY THRACIID^E. 



Foot small and grooved, not byssiferous ; without an opisthopodium, but 

 with an opisthopodial foramen in the mantle edge ; siphons long, separated to 

 their tips, naked, invertible to their tips ; mantle openings small ; monoecious, 

 marine. 



Shell earthy and cellulo-crystalline, not nacreous ; inequivalve, thin, eden- 

 tulous, often with a granular surface ; the ligament and resilium chiefly ex- 

 ternal, opisthodetic, parivincular, seated on posteriorly directed nymphae ; area 

 none, beaks usually entire ; nearly closed valves, pallial sinus present. 



Jurassic to the recent fauna. 



Ex. Thracia, Cyathodonta, Corimya, Asthenolha:rus, Bushia. 



In Asthenothcents the ligament is obsolete and the resilium internal be- 

 tween the face of the lithodesma and the valves. The subgenus Ixartia, 

 exceptionally, has the nymphs projecting into the cavity of the valves, recall- 

 ing Cochlodesma. 



FAMILY MYOCHAMIDyE. 



Gills with the external direct lamina adherent to the visceral mass ; foot 

 small, grooved ; compressed, without an opisthopodium, but with an opistho- 

 podial orifice ; pedal orifice small ; siphons moderately long, separated, the 

 branchial with a curtain valve ; marine. 



