CHAMACEA. 



731 



and is prolonged on the outside of the left valve to its beak ; and 

 the hinge is edentulous or has a single tooth. The genus is distin- 

 guished from Cha?na by its generally smooth or striated surface, and 

 the want of the foliaceous expansions of the latter. The species of 

 Reqiiienia are exclusively confined to the Cretaceous period. 



In the genus Dicer as (fig. 609) the shell is slightly inequivalve, 

 the beaks being very prominent and spirally rolled up in both valves. 



Fig. 608. — Requienia 

 ammonia.} one - fourth of 

 the natural size, a, Point 

 of attachment. Cretaceous 

 rocks. (After Woodward.) 



Fig. 6og.—Diceras arietina. Uppe 

 Jurassic. 



Either the right or the left valve may be the largest, and in that case 

 serves for the attachment of the shell to foreign bodies by means of 

 the umbo. The ligament is external, and is prolonged to the apices 

 of the beaks in external furrows. The hinge is very thick, with prom- 

 inent teeth (two in one valve and one in the other) ; and the 

 muscular impressions are bounded by long spiral 

 ridges. The species of Diceras are exclusively 

 confined to the Middle and Upper Jurassic 

 rocks, abounding especially, in some regions, in 

 beds of the age of the Coral-rag of Britain. 



In the genus Monopleura (fig. 610) the shell 

 is very inequivalve, and is fixed by the apex of 

 the right valve, which is the larger of the two, 

 and is conical in form, being either straight, or 

 more or less spirally inrolled. The left valve 

 is small, and either flat or widely conical, with 

 a submarginal umbo. The ligament is pro- 

 longed externally in grooves, which run to the 

 beaks in each valve. The surface is generally 

 radially striated. The species of this genus are 

 wholly confined to the Cretaceous rocks, and principally characterise 

 the lower division of this formation. Closely allied to Monopleura, 

 and also confined to the Cretaceous rocks, is the genus Caprotina. 



Fig. 610. — Monopleura 

 trilobata, of the natural 

 size, viewed from behind. 

 Cretaceous rocks. 



