CONCHIFERA. 



279 



they receive, and are sub-spiral, giving rise to bifid projections {c, c) on the 

 casts ; the single tooth in the left valve consists of two elements, and the 

 cavity (fosset) which receives it is divided at the bottom. 



Fig. 190. Diceras, \. Fig. 191. Requienia, §. 



Internal casts; a, point of attachment ; c, c\ casts of dental pits; /, V, furrows 



produced by spiral ridges. (Mus. Brit.) 



Requienia, Matheron. 



Dedicated to M. Requien, author of a Catalogue of Corsican Mollusca. 



Ex. R. Lonsdalii, PI. XVIII. fig. 12 and fig. 191. R. Ammonia, fig. 189. 



Shell thick, very inequivalve, attached by the left umbo; ligament ex- 

 ternal ; teeth 2:1; left valve spiral, its cavity deep, not camerated ; free 

 valve smaller, sub-spiral ; posterior adductor bordered by a prominent sub- 

 spiral ridge in each valve. 



The shell-structure of Requienia is like that of Chama. The relative size 

 of the valves is subject to much variation ; in R, Favri (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. 191, c'). 

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

 and are indicated outside by grooves (fig. 189, f). In R. sub-cequalis and 

 Toucasiana there is a second parallel ridge, as in Hippurites and Caprotina. 



Fossil, 7 sp. Neocomian — L. Chalk. Brit. France, Spain, Algeria, 

 Texas. 



FAMILY VIII. HipPURiTiD.^. 

 (Order Rudistes, Lamarck.) 

 Shell inequivalve, unsymmetrical, thick, attached by the right umbo ; 

 umbones frequently camerated; structure and sculpturing of valves dis- 

 similar ; 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. 







