440 



MANUAL OF THE il-'iLLFSCA 



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

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



The sKell structure of Reqiiienia is like that of Chama. The 

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



Fig. 229. Diceras, i. Fig. 230. Hequisnia, ^ 



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



prodace J by spiral ridges. (Mus. Brit.) 



Z^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 dupHcatures 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 CaproUna. 



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

 Spain, Algeria, Texas. 



Family VIII. — Hippuritid^. 

 (Order Budistes, Lamarck.) 



Shell inequivalve, uns3Tnmetrical, thick, attached by the 

 right umbo ; umbones frequently camerated ; structure and 

 sculpturing of valves dissimilar ; ligPvment 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 Eui'ope, 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 

 sheila has been fully described in the Quarterly Journal of the 



