542 TRANSACTIONS OF THE WAGNER FREE 



sessile ; closed, usually rounded in form with conspicuous sculpture, often differ- 

 ing in the opposite valves ; adductor scars subequal, elongate, pedal scars mi- 

 nute, distant; ligament and resilium external in a deep groove, parivincular, 

 opisthodetic ; area distinct, prosodetic ; beaks more or less spiral, prosogyrous ; 

 pallial line simple; hinge-plate heavy, arcuate; hinge frequently with a minute 

 or obsolete posterior lamina, chiefly in the fixed valve ; cardinals one or two in 

 the free valve, two with an intermediate socket in the fixed valve ; the anterior 

 cardinal broad, usually deeply grooved or multifid, the posterior simple, long, 

 and curved parallel with the dorsal border. 

 Cretaceous to recent fauna. 



Ex. Chama, Echinochania. 



Either of the valves of Chama may be the sessile one, but the teeth in the 

 fixed valve, whether right or left, are always the same, and similarly with the 

 free valve. 



Family DICERATID.^. 



Resembling Chama, but with the adductors usually borne on myophoric 

 laminae, or projections which are prolonged into the umbonal cavity below the 

 hinge-plate ; the valves are grotesquely distorted, subequal, with prolonged 

 and twisted umbones, or the free valve is reduced to an opercular form, spiral 

 and even concave ; the teeth are often reversed relatively to their situation in 

 Chama ; marine. 



Jurassic and Cretaceous. 



Ex. Diceras, Heterodiceras, Reguienia, Toucasia, Apricardia. 



Family MONOPLEURID^. 



Shell substance without canals ; shell sessile, closed, very inequivalve ; free 

 valve with the cardinal formula loi, operculiform or slightly spiral; fixed 

 valve with the formula oio, conical, unrolled, or spiral; area none; ligament 

 external, parivincular, opisthodetic ; marine. 



Cretaceous. 



Ex. Monopleura, Valletta, Bayleia. 



Family CAPRINID^. 



Shell substance internally furnished with large parallel canals, the exter- 

 nal layer prismatic ; valves heavy, irregular, unequal, closed ; free valve spiral, 

 cardinal formula lOi, with a posterior myophoric crest for the adductor; fixed 

 valve conical or spiral, cardinal formula oio ; ligament in a deep groove, some- 

 times almost internal, parivincular, opisthodetic ; marine. 



Cretaceous. 



Ex. Plagioptychus, Caprina, Ichlhyosarcoliles, Caproiina, Polyconiles. 



