institute of science, philadelphia. 531 



Family ANATINID^. 



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 ; monoecious, marine. 



Jurassic to the recent fauna. 

 Ex. Anatina, Periplomya, Cercornyd, Rhynchomya, Anaiimya, Plaiymya. 



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, Bonttsa, ? Tyleria. 



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



Family THRACIID.^. 



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 nymphs ; area 

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



Jurassic to the recent fauna. 

 Ex. Thracia, Cyathodojita, Corimya, Asthenothcerus, Bushia. 



In Asthenothcerus the ligament is obsolete and the resilium internal be- 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 MYOCHAMID^. 



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. 



