CLASS I PELECYPODA 477 



with an oiierculiforni free valve. The higlily modified Hippuritidae evidently indicate the 

 last stage of the evolutioiiary series. 



Diceras Lam. {Heterodiceras, Plesiodiceras Mun.-Chalm. ; Pseudodiceras Gemm.) 

 (Figs. 765, 766). Shell smooth, inequivalve, with botli valves convex, the attached 

 valve larger, dentition normal or inverse ; beaks prominent, j^rosocoelous ; ligament as 

 in Chama, supported on nympliae ; right valve with a small anterior and large 

 elongated curved posterior tootli almost parallel with the hinge margin ; left valve 



A, Rcquicnia ammonia (Goldf.). Urgonian ; Orgon, Bouches-du-Rli6ne. 1/3. B, C, Small individual of 

 R. {Toueasia) lonsdalei (Sow.), from same locality. B, Left ; C, Right valve, Vi- 



with a Single, large, ear-shaped tooth in front of the elongated socket for the principal 

 tooth of the right valve ; posterior adductor scar on a projecting buttress. Upper Jura. 



Apricardia Gueranger. Cenomanian and Turonian. A. carinata Guer. 



Requienia Matheron (Fig. 767, A). Smooth, very inequivalve, attached by the 

 spirally twisted beak of the left valve ; right valve opercular, spiral, flat ; teeth 

 feeble ; posterior adductor scar buttressed. Lower Cretaceous, especially the Urgonian 

 of Southern Europe, the Alps and Texas. 



Subgenus Toueasia Mun.-Chalm. (Fig. 767, B, C). Differs from Rcquicnia in having 

 both valves keeled. Urgonian and Cenomanian. 



Matheronia Mun.-Chalm. Urgonian and Cenomanian. M. vircjinae (Gras.). 



Family 10. Ohamidae Lamarck. 



Bhell suhstance threefold, the inner layers porcellanoiis and tubulär, the middle 

 obscurely prismatic, the external cellulo-crystalline with reticulated tuhules and an incon- 

 spicuous epidermis ; valves unequal, irregulär, one of them sessile ; closed, usually 

 rounded in form with conspicuous sculpture, often differing in the opposite valves ; 

 adductor scars suh-equal, elongate, pedal scars minute, 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, chießy in the fixed valve; 

 cardinals one or two in the free valve, two with an intermediate socket in the ßxed valve ; 

 the anterior cardinal broad, usually deeply grooved or multißd, the posterior simple, long 

 and curved parallel with the dorsal border ; siphonal orifices not produced into tubes ; 

 adductors each composed of tivo Clements. Cretaceous to Receiit. 



Either of the valves of Chama may be the attached one, but the teeth in the fixed valve, 

 whether right or left, are ahvays the same, and similarly with the free valve. The fixation 

 is generally by the left valve. 



Chama lAnw. (Fig. 768). Nepionic shell rounded. Ligament sometimes continued 

 to the point of the beaks, as in otlier bivalves with g}^rate umbones ; form rounded, 



