478 



MOLLUSCA 



PHYLUM VI 



attached valve deeper and larger, the free valve flatter ; margins usually cross-striated, 

 surface lamellar or spinose ; adductor scars large, not elevated. Cretaceous to Recent; 

 maximum in Eocene. 



Fifi. 768. 

 Cliwma squamosa Lam. Eocene ; Hamp-sliire. i/i. 



Echtnochama Fiscli. Nepionic shell elongated, liaving the form, hinge and other 

 characters of Cardita ; attached when adolescent, free in the young and adult stages. 

 Valves sub-equal and similar; surface vermiculate, sj)inose, with radial ribs. Oligocene 

 and Recent. 



Family 11. Monopleuridae Fischer. 



Shell substance ivithout canals; shell sessile, closed, very inequivalve ; free valve with 

 the cardinal formula 101, opcrculiform or slightly spiral; fixed valve with the formula 

 010, conical, unrolled or spiral; area loanting ; ligament external, parivincular, 

 opisthodctic. Cretaceous. 



Monopleura Math. (? Dipilidia Math.) (Figs. 769, 770). Very inequivalve, smooth 

 or ribbed ; dentition always inverse ; attached by the right valve, which may be either 



Fio. 769. 



Fig. 770. 



Monopleura trilohita d'Orb. Neocomian (Schiiittenkalk) ; Orson, Monopleura varians Matli. Ur- 



Bouches-du-Rhone. l/j. A, B, Anterior and posterior views. C, Interior gonian ; Orgon, Bouclies-du-Uli6ne. 

 attached valve. Interior of both valves, i/j. 



twisted or coniform ; left valve conical or flat ; ligament as in Chama ; posterior 

 adductor scar buttressed. Lower Cretaceous ; Southern Europe and Texas. 



Valletia Mun.-Clialm. Neocomian. Gyropdeihra Douvilk'. Cenomanian to 

 Senonian. Baylcia Mun.-Chalm. Turonian. B. pouechi Mun.-Chalm, 



Family 12. Oaprinidae d'Orbigny. 



Shell substance internally furnished ivith large parallel canals, th.e external layer 

 prismatic ; valves heavy, irregular, unequal, closed ; free valve spiral, cardinal formula 



