440 



MAinJAL OF THE MOLLTTSCA. 



not camerated ; free valve smaller, sub- spiral ; posterior ad- 

 ductor bordered by a prominent sub-spiral ridge in each, valve. 



The shell structure of Eequienia is like that of Cliama. The 

 relative size of the valves is subject to much variation ; in R. 



Fig. 229. Diceras, |. Fig. 230. Eequienia, f 



Internal casts ; a, point of attachment ; c, c\ casts of dental pits ; t, t', furrows 



produced by spiral ridges. (Mus. Brit. ) 



Ji^avn'(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, c'). The internal 

 muscular ridges are produced by duplicatures of the shell-wall, 

 and are indicated outside by grooves (Fig. 229, t'). In R. sub- 

 (squalis and Toucasiana there is a second parallel ridge, as in 

 Hippurites and Caprotina. 



Fossil, 7 species. Neocomian — L. Chalk. Britain, France, 

 Spain, Algeria, Texas. 



Family YIII. — ^Hippubitid^. 

 (Order Rudistes, Lamarck.) 



Shell inequivalve, unsymmetrical, thick, attached by the 

 right umbo ; umbones frequently camerated ; structure and 

 sculpturing of valves dissimilar ; ligament internal ; 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 of the Peninsula, 

 the Alps, and Eastern Europe, where the equivalent of the 

 Lower Chalk has received the name of "Hippurite limestone." 

 They occur also in Turkey and in Egypt, and Dr. F. Eoemerhas 

 found them in Texas and Guadaloupe. The structure of these 

 shells has been fully described in the Quarterly Journal of the 



