INSTITUTE OF SCIENCE, PHILADELPHIA. 54! 



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

 and prolonged posterior cardinal tooth. 



FAMILY CARDITIDyE. 



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 AstartidiZ, but with usually radial sculpture, the pedal adjacent 

 to the anterior adductor scar; ligament external, parivincular; resilium 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 forrqula KioToi- 



Trias? Jura to recent fauna. 



Ex. f Pachycardia, Beguina, Pleuromeris, Carditel/a, Calyptogena, Cardita, Venericardia, 

 Cardilamera, 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In some of the minuter Austral forms the ligament is obsolete or subin- 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 CHAMACEA. 



Carditian forms specialized for a sessile habit, usually with exceptionally 

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

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



FAMILY CHAMID^E. 



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 



