554 TRANSACTIONS OF THE WAGNER FREE 



anterior lamina approximate, posterior more distant from the cardinals, when 

 present ; cardinal teeth small. 

 Jurassic to recent fauna. 

 Ex. Tettina, Tellidora, Strigilla, Gaslrana, Macoma, Capsa, etc. 



FAMILY SEMELID^E. 



Resembling Tellinidce, but with the resilium internal, often on a distinct 

 chondrophore, and with the laterals, when present, stronger and less distant. 



Tertiary to recent fauna. 

 Ex. Semelc, Scrobicularia, Abra, Cumingia, Theora, Souleyetia, Endopleura, fOedalina. 



FAMILY PSAMMOBIIDyE. 



Resembling Tellinidce, but with the gills plicate, the outer direct and re- 

 flected limb ventrally directed ; the palpi smaller in proportion to the gills. 



Shell as in TeUinida, but usually more equivalve and less twisted, with 

 more conspicuous epidermis, broader hinge-plate and more conspicuous 

 nymphs, a wider posterior gape, absence of lateral laminae on the hinge and 

 the cardinals sometimes reaching three in one valve ; the ligament is external 

 and conspicuous, and there is no defined area. 



Tertiary to recent fauna. 

 Ex. Ptiorhytis, Asaphis, Psammobia, Hiatula, Sanguinolaria, Elizia, Heterodonax. 



FAMILY DONACID^l. 



Resembling Psammobiida ; but with shorter and stouter siphons, smaller 

 palpi, and proportionately larger gills, which are united behind and to the si- 

 phonal septum forming a complete anal chamber ; foot very large, compressed 

 and sharp-edged, not byssiferous; dioecious, marine. 



Valves equal, free, subtrigonal, usually closed, solid ; outer surface and inner 

 margins smooth or radially sculptured, the posterior end usually shorter and 

 obliquely subtruncate ; pallial sinus similar in both valves ; resilium subinter- 

 nal, sometimes amphidetic ; ligament short, external, seated in a deep groove, 

 opisthodetic ; hinge-plate moderately developed, with, usually, a posterior and 

 anterior lateral in the right and corresponding sockets in the opposite valve ; 

 cardinal formula jnn4~J, the strongest cardinal tooth often bifid. 



Liassic to recent fauna. 

 Ex. Isodonta, Oncophora, ? Egeria, Donax, Iphigenia. 



The resilium is chiefly opisthodetic and subinternal, but some of the 

 large species have a small segment of the resilium separate from the rest, wholly 

 internal and in front of the beaks. This is due, perhaps, to the excessive um- 

 bonal concentration of the hinge elements and may be noted also in some spe- 

 cies of Circe. I regard it as a secondary adaptation and not an archaic reten- 

 tion. Heterodonax, by its anatomy as well as its shell characters, is better 

 placed in Psammobiidce. The typical Donaces date only from the later Eocene. 



