72 PALEONTOLOGY. 



measures and older rocks are extremely problematic, and may 

 even belong to marine genera. 



Of the genus Chama there is one species in the upper 

 greensand and chalk of England, and another in the London 

 clay. Elsewhere they are more abundant, amounting to thirty 

 species. Closely allied to Chama is the Dicer as (Lam.), of 

 which the remarkable casts attracted attention at an early 

 period (fig. 18, i). They are found in the coral rag of France 

 and Germany, and resemble the horns of some animal. The 

 shell is attached by the umbo of either valve, indifferently, like 

 some of the recent Chamas. The posterior adductor muscle 

 is supported on a prominent ridge (as in Pachydesma, Mega- 

 lodon, and the recent Cardilia), which causes a spiral furrow 

 in each horn of the cast. The shells which succeed Diceras, 

 in the lower cretaceous strata, have the right valve usually 

 much smaller than the left, and in one instance (fig. 18, 2) it 

 is like the operculum of a spiral univalve. The only British 

 species of this group is Reqtcienia Lonsddlii, found in the 

 ironsand of Bo wood. In France, and also in Texas, another 

 form occurs, with the attached valve simple and conical, like 

 a Hippurite. The ligamental groove is straight, and the umbo 

 of the free valve marginal. 



These shells are so intimately allied to the Hippuritidw, 

 that Requienia has been frequently included with them in 

 the apocryphal order " Eudista." The members of the Hippu- 

 rite group are attached and gregarious, like oysters, often 

 occurring in great numbers, and filling large tracts of rock. 

 Their valves are different in structure and sculpturing, and 

 are articulated by two prominent teeth above and one below ; 

 the cartilage is internal, but there is a conspicuous ligamental 

 furrow outside. There are nearly 100 species characteristic 

 of the cretaceous strata, and especially of the lower chalk, or 

 " hippurite limestone." Only two species (Radiotites Mortoni 

 and Caprinella triangularis) are found in England ; the rest 



