54 TRANSACTIONS OF THE WAGNER FREE 



Superfamily CYRENACEA. 



Cypricardians which have become specialized for fresh or brackish water 

 conditions and, as usual in such cases, have developed great variability of 

 character. 



FAMILY CYRENID^. 



Anatomy as in Venie/litue, except that the mantle is more open ventrally, 

 the siphons distinctly developed, short, united more or less, usually with papil- 

 lose orifices ; dioecious ; fluviatile or estuarine. 



Shell porcellanous, with a conspicuous epidermis, usually with concen- 

 tric sculpture ; valves equal, free, closed, usually with plain margins ; area ob- 

 scure or none ; ligament and resilium external, parivincular, opisthodetic ; ad- 

 ductor scars subequal, separate from the pedal ; pallial line simple or with a 

 sinus ; hinge with anterior and posterior lamina: usually double in the 

 right, single in the left valve, distinctly separated from the cardinals ; cardinal 

 teeth bifid at the summit, three in each valve when none are obsolete. 



Lias to recent fauna. 



Ex. Loxoptychodon, Donacopsis, Ditypodon, Veloritina, Leptesthes, Miodon (Sandb ) Cv 

 rcna, Neocorbicula, Corbicula, Velorita, Batissa. 



Many of these forms merge with one another as we recede in time. The 

 recent American forms and many fossils show a pallial sinus. Oriental species 

 are generally without it. In some fossils the lamina; of the right valve are not 

 double. 



FAMILY 



Anatomy much as in Cyrenidce, except that the siphons are separate and 

 plain, the branchial sometimes not complete below ; the foot prolonged ventral- 

 ly, narrow, grooved, byssiferous when young ; moncecious, the nepionic young 

 incubated in a marsupium formed by the inner limb of the ctenidia ; confined 

 to fresh water. 



Shell as in Cyrenid<z, but small, with a feeble, short ligament, a simple 

 palhal line, no hinge-plate ; the cardinal teeth (usually two in each valve) vari- 

 able, very thin, often nearly parallel to the hinge-margin or defective in part of 

 the series ; the laterals as in Cyrenidce, distinct; the nepionic stage of the shell 

 often conspicuous on the beaks. 



Eocene to recent fauna. 

 Ex. Spharium, Eupera, Pisidium. 



In Splunriiim the branchial siphon is complete, in Pisidium merged with 

 the pedal opening. If this is found by more extensive observations to be con- 

 stant within the genera, the family might be divided into two subfamilies. 



Superfamily CARDITACEA. 



This group seems to have branched off from the Astartoid radical in the 

 early Mesozoic, forming in one sense a sort of parallel series with the Astartida, 



