LAMELLIBRANCIIIATA 63 



period (fig. 14, 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 Pachydcsma, 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. 14, 2) it 

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

 species of this group is Requicnia Lonsdalii, found in the 

 ironsand of Bowood. 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 Hvppuritidce, 

 that Requicnia 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 one species (Radiolites Mortoni) 

 is found in England ; the rest are from the West Indies, 

 Southern Europe, Algeria, and the East. The form which 

 approaches nearest to Chama is the little genus Caprotina (fig. 

 15, 7), whose upper valve has a marginal umbo, but is in other 

 respects like a miniature Radiolite. Caprina (d'Orb.) has 

 the free valve perforated by canals which open in the inner 

 margin, and in Caprinclla the outer lamina of both valves 

 possesses this structure. One valve is sometimes spiral (fig. 



