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. Tellina, Tellidora, Slrigilla, Gaslrana, Macoma, Capsa, etc. 



Family SEMELIDyE. 



Resembling Tellmidcs, 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. Semele, Scrobicularia, Abra, Cuniing-ia, Theora, Souleyetia, Endopleura, fOedalina. 



Family PSAMMOBIID.^. 



Resembling TellinidcB, 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 Tellinidcs, 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. Pliorhylis, Asaphis, Psammobia, Hiaiula, Sanguinolaria, Elizia, Heierodonax. 



Family DONACID^. 



Resembling /!ya;«;«i3i^zV(f(^, 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 j^ [, ° ^ ° , the strongest cardinal tooth often bifid. 



Liassic to recent fauna. 

 Ex. Isodonia, 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. 



