CLASS I 



PELECYPODA 



477 



with an operculiform free valve. The highly modified Hippuritidae evidently indicate the 

 last stage of the evolutionary series. 



Diceras Lam. (Heterodiceras, Plesiodiceras Mun.-Chalia. ; Pseudodiceras Gemiu.) 

 (Figs. 765, 766). Shell smooth, inequivalve, with Ijotli 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 tooth almost parallel with the hinge margin ; left valve 



Fk;. 767. 



A, liequienia atiimomaifioldt'.). Urgouiaii ; Orgon, Boiu;heH-du-Rliiine. i/s- /'', f. Small individual of 

 11. (ToHCJxaia) lonxihde! (Sow.), fioin same locality. B, Left ; <', Kiglit valve, i/j. 



with a single, large, ear-shajjed tooth in front of the elongated socket for the principal 

 tooth of the right valve ; posterior addnctor scar on a projecting Inittress. 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 Toucasia Mun.-Chalm. (Fig. 767, B, C). Ditfers from Requienia in having 

 both valves keeled. Urgonian and Cenomanian. 



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



Family 10. Chamidae Lamarck. 



Shell substance threefold, the inner layers imrcellanous and tubnlar, the middle 

 obscurely prismatic, the external cell lolo -crystalline ivith reticulated tubules and an incon- 

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

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

 adductor scars sub-eqiial, 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 ivith a minute or obsolete posterior lamina, chiefly in the fixed valve; 

 cardinals one or two in the free valve, tioo loith an intennediate socket in the fixed valve ; 

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

 and curved parallel ivith the dorsal border ; siphoned orifices not produced into tubes; 

 adductors each composed of txoo elements. Cretaceous to Recent. 



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

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

 is generally by the left valve. 



Chama Linn. (Fig. 768). Nepionic shell roimded. Ligament sometimes continiied 

 to the point of the beaks, as in other bivalves with gyrate umbones ; form rounded. 



