INSTITUTE OF SCIENCE, PHILADELPHIA. 54I 



with which it is contrasted most obviously by its prevailingly radial sculpture 

 and prolonged posterior cardinal tooth. 



Family CARDITID^. 



Anatomy resembling Astartidce, but the gills united behind the foot and 

 of a very simple reticulate type; foot usually byssiferous ; usually dioecious, 

 marine. 



Shell as in Astartidcs, but with usually radial sculpture, the pedal adjacent 

 to the anterior adductor scar; ligament external, parivincular; resiliuni usually 

 included in the ligament, rarely internal; fully-developed hinge, with the lam- 

 inae as in Astartidce, and usually obsolete; the anterior cardinal often obsolete, 

 the posterior prolonged parallel with the dorsal margin even below the liga- 

 ment. Full cardinal formula rj- q \W . 



Trias ? Jura to recent fauna. 



Ex. f Pachycardia, Begidna, Pleuromeris, Carditella, Calyptogena, Cardita, Venericardia, 

 Cardiiamera, Thecalia, Milneria. 



Some of these groups have a shelly marsupium in the female shell. In . 

 the triangular species, and especially those with strongly-curved umbones, 

 there is a tendency for the resilium to descend into the hinge-plate and become 

 more or less internal. 



In some of the minuter Austral forms the ligament is obsolete or subin- 

 ternal, and the resilium wholly internal. A graduated series of species may 

 be selected, showing the progress of immersion, rendering it difficult to draw 

 a sharp line of separation between these and more normal forms. 



In the cases where the ligament is most feeble, the lateral laminae con- 

 trariwise are most efficiently developed. 



Superfamily CHAM ACE A. 



Carditian forms specialized for a sessile habit, usually with exceptionally 

 spiral growth, and very unequal valves. Echinochama has a free nepionic 

 stage in which it has the form, hinge and other characters of Cardita. 



Family CHAMID^. 



Gills plicate, the outer limb smaller and appendiculate, united behind to 

 each other and the siphonal septum, forming an anal chamber ; palpi normal ; 

 foot small, not byssiferous; lobes of the mantle united to form rather distant 

 anal, branchial, and pedal orifices ; mantle margin papillose, siphonal orifices 

 not produced into tubes ; the ovary extensively distributed in the mantle 

 lobes; adductors each composed of two elements; dioecious, marine. 



Shell substance three-fold, the inner layer porcellanous and tubular, the 

 middle obscurely prismatic, the external cellulo-crystalline with reticulated 

 tubules and an inconspicuous epidermis; valves unequal, irregular, one of them 



