ASIPHONIDA. 



475 



former. The genera Entolium, of the Carboniferous, and 

 StrebbpteriOj of the Carboniferous and Permian, comprise 

 forms more or less closely related to the preceding. 



A fourth group of the Ostreidce, also often regarded as a 

 distinct family (Limadce), is that typified by the genus Lima 

 or Radula. In this genus the 

 shell is equivalve and free, and 

 the beaks are separated from 

 one another and eared (fig. 

 331). The surface is generally 

 adorned with radiating ribs, 

 and there is a median cartilage- 

 pit, and a triangular hinge-area. 

 The genus (including Plagio- 

 stoma) appears to occur in the 

 Carboniferous, is abundantly 

 represented in the Triassic, 

 Jurassic, Cretaceous, and Ter- 

 tiary rocks, and exists in dimin- 

 ished numbers at the present 

 day. Limcea, ranging from the 

 Trias to the Eecent period, differs from Lima principally 

 in having the hinge-line closely toothed ; and the Limoptera 

 of the Devonian may be an early representative of this group. 



A fifth group of the Ostreidce, sometimes raised to the 

 rank of a distinct family (Spondylidce), is that represented by 

 the Thorny Oysters, forming the genus Spondylus. In this 



Fig. 331. Lima (Plagiostoma) gigantea. 

 Lias. 



Fig. 332. Spondylus spinosus. Chalk. 



genus (fig. 332) the shell is inequivalve, and is fixed by the 

 right valve to some foreign body. The beaks are apart and 



