552 TRANSACTIONS OF THE WAGNER FREE 



FAMILY VENERID^E. 



Outer gills smaller, frequently with a dorsal appendix; palpi moderate, 

 free behind ; foot compressed, keeled, short, in the typical forms not grooved 

 or byssiferous; mantle edge duplex, papillose; siphons developed, naked, 

 moderately long, united more or less, with papillose orifices, the anal with a 

 distal valve, the branchial larger with a small curtain valve proximally ; cli- 

 cecious, marine. 



Valves equal, free, closed, with prosogyrous beaks, variably sculptured, 

 with the margins more or less dentate, except in the smooth species ; adductor 

 scars peripheral, pedal distant ; pallial sinus more or less sinuated ; area very 

 distinct ; resilium usually external, embraced by the ligament ; hinge-plate de- 

 veloped ; formula of the cardinals usually ^-A|I, \\Jth a single obsolete 

 lateral in one valve, the cardinals frequently bifid, usually radially disposed and 

 subequal in size, except the posterior left one, which is often obsolete or obscure ; 

 supplementary cardinals or rugosities are present in specialized forms. 



Jurassic to recent fauna. 



The family must be divided into at least four subfamilies, as follows : 



Veuerinat; typical, with produced siphons, not byssiferous, the young not 

 retained within the mother after leaving the e?p 



o o o 



Ex. Pronoella, Cyprimeria, Dosima, Cyclina, Lucinopsis, Meretrix. Tivela, Gratelupia, 

 Sunetta, dementia, Venus, Veneriglossa, etc. 



Circina ; with separate short siphons, correlative nearly simple pallial 

 line, subinternal, partially amphidetic resilium, and compressed beaks. 

 Ex. Plychomya, Circe, Gouldia. 



Tapetina: ; with long but partly separated siphons, a developed byssus, 

 at least in the young; hinge with no lateral teeth, otherwise like Vcnerince. 

 Ex. Baroda, Venerupis, Tapes, Saxidomiis, Liocyma. 



Gemmince ; minute shells, with more or less separated siphons, no byssus, 

 and obsolete lateral lamina.- ; sheltering the nepionic young within the cavity of 

 the mother. 



Ex. Gemma, Parastarte, Psephis. 



Much still remains to be learned about the anatomy of the different forms 

 of Venerida and the above arrangement is tentative. 



FAMILY PETRICOLID/E. 



Animal resembling Tapes, with long siphons united only near the base, 

 not byssiferous, the outer reflected limb of the gill with an appendix ; pallial 

 line sinuated ; dicecious, marine, chiefly boring animals. 



Valves, when not distorted, equal, free, somewhat gaping behind, nuliately 

 sculptured with plain margins and inconspicuous beaks; posterior adductor 

 scar larger than the anterior; pedal narrow, elongated, distinct; ligament and 



