400 MOLLUSCA SUB-KINGDOM VI 
grotesquely distorted, sub-equal, with prolonged and twisted umbones, or the free valve ds 
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 
Fic. 718. 
_A, Diceras arietinum, Lam. Coral-Rag ; St. Mihiel, Meuse. 
Fixed left valve, 2/3. B, D. Zitteli, Mun.-Chalm. Tithonian ; 
Stramberg. Right valve, 2/s. a, a’, Anterior and posterior 

Fic. 71%. adductor scars; c, Major cardinal; d, Socket for left anterior 
Diceras arietinum, Lam. Coral - Rag ; cardinal; 7, Ligamentary groove; s, Buttress ridge before 
St. Mihiel, Meuse. 2/3. posterior adductor sear. 
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 
A 

Fic. 719. 
A, Requienia ammonia, Goldf. Urgonian; Orgon, Vaucluse. 1/3. B, C, Small individual of R. (Toucasia) 
Lonsdalei, Sow. sp., from same locality. B, Left; C, Right valve, 1/). 
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, Guéranger. Cenomanian and Turonian. 
Requienia, Mathéron (Fig. 719, A).' Smooth, very inequivalve, attached by the 
v | ) § ) ’ 4 1 ’ W 
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. 
b ”) 
Sub-genus Youcasia, Mun.-Chalm. (Fig. 719, B, C). Differs from Requienia in having both 
valves keeled. Urgonian and Cenomanian. 
Matheronia, Mun.-Chalm. Urgonian and Cenomanian. 
