400 



MOLLUSCA 



SUB-KINGDOM VI 



grotesquely distorted, sub-equal, with prolonged and twisted umbones, or the free valve is 

 reduced to an opercular form, spiral, and even concave; the teeth often reversed relatively 

 to their situation in Chama. Jura and Cretaceous. 



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

 (Figs. 717, 718). Shell smooth, inequivalve, with both valves convex, the attached 



B 



FIG. 717. 



Diceras arietinum, Lam. Coral - Rag ; 

 St. Mihiel, Meuse. 2/ 3 . 



A, Diceras arietinum, Lam. Coral-Rag ; St. Mihiel, Meuse. 

 Fixed left valve, 2/ 3 . B, D. Zitteli, Mun.-Chalm. Tithonian ; 

 Stramberg. Right valve, ~/ 3 . a, a', Anterior and posterior 

 adductor scars ; c, Major cardinal ; d, Socket for left anterior 

 cardinal ; ?, Ligamentary groove ; s, Buttress ridge before 

 posterior adductor scar. 



valve larger, dentition normal or inverse ; beaks prominent, prosocoelous ; ligament as 

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

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



FIG. 719. 



A, Requienia ammonia, Golclf. Urgonian ; Orgon, Vaucluse. 

 Lonsdalei, Sow. sp., from same locality. B, Left ; C, Right valve, 



-B> C, Small individual of R. (Toucasia) 



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. 



Requienia, Matheron (Fig. 719, 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. 



Sub-genus Toucasia, Mun.-Chalm. (Fig. 719, B, 0). Differs from Requienia in having both 

 valves keeled. Urgonian and Cenomanian. 



Matheronia, Mun.-Chalm. Urgonian and Cenomanian. 



