440 MANUAL OF THE MOLLUSCa 
not camerated; free valve smaller, sub-spiral; posterior ad- 
ductor bordered by a prominent sub-spiral ridge in each yalve. 
The shell structure of Requienia is like that of Chama. The 
relative size of the valves is subject to much variation ; in &. 
Fig. 229, Diceras, t. Fig. 280. Keqgutenza, 4 
Tnternal casts; @, voint of attachment; c, c’, casts of dental pits; 7, ¢’, furrows 
produced by spiral ridges. (Mus. Brit.) 
Favrt (Sharpe) they are nearly equal. The hinge-teeth are like 
those of Diceras ; the cavity for the posterior tooth of the right 
valve is very deep and sub-spiral (Fig. 230, ¢). The internal 
muscular ridges are produced by duplicatures of the shell-wall, 
and are indicated outside by grooves (Fig. 229, ¢’). In &. sub- 
equalis and Toucasiana there is a second parallel ridge, as in 
Hippurites and Caprotina. 
Fossil, 7 species. Neocomian — lL. Chalk. Britain, France, 
Spain, Algeria, Texas. 
Famity VIII.—HIPPuritTip”. 
(Order Fudistes, Lamarck.) 
Shell imequiyalye, unsymmetrical, thick, attached by the 
right umbo; umbones frequently camerated; structure and 
sculpturing of valves dissimilar; ligamentinternal ; hinge-teeth 
1:2; adductor impressions 2, large, those of the left valve on 
prominent apophyses; pallial line simple, sub-marginal. 
The shells of this extinct family are characteristic of the 
cretaceous strata, and abound in many parts ofthe Peninsula, 
the Alps, and Hastern Europe, where the equivalent of the 
Lower Chalk has received the name of ‘‘ Hippurite limestone.” 
They occur aiso in Turkey and in Egypt, and Dr. F. Roemer hae 
found them in Texas and Guadaloupe. The structure of theze 
snells has been fully described in the Quarterly Journal of the 
