5 22 TRANSACTIONS OF THE WAGNER FREE 



FAMILY CARDINIIDyE. 



Shell equivalve, closed, with feeble concentric sculpture or smooth ; di- 

 myarian ; ligament opisthodetic, external ; adductor scars subequal, pedal scars 

 feeble or invisible ; station marine or brackish water. 



Devonian to the Trias. 



Ex. Cardinia, Anthracosia, Carbonicola, Anoplophora, Guerangcria, Nyassa, etc. 



This group has been assimilated to the Unionidee, which their schizodont 

 dentition strongly recalls, and it is possible that the resemblances indicate 

 either the origin of the Unionidee or a parallel line of development from Pterii- 

 da adapted to a marine station. So far as known they were not distinctly 

 nacreous, which, if confirmed, does not favor intimate relationship with the 

 Unionidee. 



? FAMILY MEGALODONTIDyE. 



Shells equivalve, submytiliform, closed, with feeble concentric sculpture 

 or none; dimyarian, with amphidetic area, and external opisthodetic ligament, 

 frequently supported by nymphs ; anterior adductor scars distinct, with a well- 

 marked myophoric ridge and pedal scar, the posterior adductor scars frequently 

 bordered by an elevated crest ; station marine. 



Silurian to the Cretaceous. 



Ex Megalodon, Megalomus, Pachyrisma, Durga, Dicerocardium. 



These shells, which are often very ponderous, sometimes bear a remark- 

 able resemblance to some recent American Uniones. The myophoric ridge is 

 common to very distinct bivalves of many unrelated groups. 



FAMILY UNIONIDyE. 



Shell equivalve, dimyarian, typically schizodont, with pseudocardinals and 

 laterals if dentiferous ; conspicuously nacreous; the beaks usually sculptured; 

 ligament opisthodetic, external ; gills attached to the mantle, forming an incom- 

 plete septum ; either or both pairs modified as a marsupium ; lobes of the man- 

 tle united to form an anal siphon, but the functional branchial siphon usually 

 incomplete below ; foot compressed, keeled, large, rarely with a feeble byssus ; 

 usually dioecious ; the young having a distinct nepionic stage (glochidium), 

 during which they are encysted on the fins or gills of fresh-water vertebrates ; 

 station fluviatile. 



Carboniferous or Jura to the recent fauna. 



Ex. ? Anthracomya, Unio, Limnium, Anodon, Arconaia, Hyria, Metaptera, Castalia. 

 (='fetraplodoii), Casta/ina, Cristaria, Pseudodon, Solenaia, etc. 



Some Uniones have a functional byssus of a few threads, and there are 

 always two functional siphons, though the branchial opening is usually incom- 

 plete ventrally. In some species there are one or more small openings in the 

 dorsal mantle margin above and nearer to the beaks than the true anal open- 



