55 8 TRANSACTIONS OF THE WAGNER FREE 



Shell small, but much as in Mya, but the pallial sinus feeble or obsolete; 

 the ligament usually stibexternal, separated from the resilium, which is inter- 

 nal, alivincular and amphidetic ; the chondrophore is received into a socket of 

 the opposite valve, not merged with the valve margin ; hinge with one or two 

 subumbonal projecting teeth, and rarely obscure traces of laterals ; the poste- 

 rior gape inconspicuous. 



Triassic to the recent fauna. 



Ex. Paramya, ? Pleuromya, Corbulamella, Corbulomya, Anisolhyris, Himella, Bothrocor- 

 bula, Corbula, Erodona, ? Basterotia. 



The Cretaceous Corbulamella has an anterior myophore ; Basterotia is ex- 

 ceptional in having the ligament external, on distinct nymphs, but may not 

 belong to this family. The most salient shell character of this group is the 

 strong dentiform process in front of the chondrophore. 



FAMILY SAXICAVID^. 



Resembling Mya, but the gills extended into the cavity of the branchial 

 siphon; foot small, digitiform, grooved, byssiferous when young; siphons 

 united, not wholly retractile, generally tunicate ; mantle lobes smooth-edged, 

 largely united (sometimes with an opisthopodial orifice ?) ; dioecious, marine. 



Shell substance as in Mya ; epidermis conspicuous ; valves equal, free, rude, 

 and often irregular, more or less elongated and gaping, not fully covering the 

 animal ; adductor scars often irregular, the pallial line discontinuous or irregu- 

 lar, the sinus distinct ; shell margins smooth ; area obsolete ; ligament and resil- 

 ium external, parivincular, seated on strong nymphs, sometimes widely ex- 

 tended ; hinge without laterals, with few feeble or obsolete subumbonal cardinals. 



Cretaceous to recent fauna. 



Ex. Glycimeris, Saxicava, ? Cyrlodaria. 



Hancock refers to an opisthopodial orifice in " Panopea," but I have found 

 none in P. generosa Gld. In Cyrtodaria the pedal orifice is anterior, the hinge 

 edentulous, and the epidermis leathery, while the others have a ventral pedal 

 orifice, obscure hinge-teeth, and a papery epidermis. 



FAMILY GASTROCH/ENID^ ? 



Gills as in Saxicavida, prolonged into the branchial siphon ; foot digitiform, 

 small, not grooved or byssiferous ; mantle edges thick, smooth, united, with a 

 small pedal foramen; siphons naked, united, not wholly retractile; dioecious, 

 marine. 



Shell substance as in Saxicava ; valves equal, widely gaping in front; 

 adductor scars unequal, the anterior smaller; pallial sinus deep, margins sim- 

 ple"; area none; ligament and resilium external, parivincular; hinge with a sin- 



