PECTIN ACE A. 699 



is subcentral. The two principal genera of this family are Spon- 

 dylus and Plkatula. 



The genus Spondylus, comprising the " Thorny Oysters," has an 

 equivalve shell, the right valve being the deepest, and serving for 

 the attachment of the shell to foreign bodies (fig. 563). The beaks 

 are separated and are eared, and the shell is covered with spines, 

 foliaceous expansions, or ribs radiating from the beak. The lower 

 valve has a triangular hinge-area, and there are two teeth in each 

 valve. The Spondyli seem to have commenced in the Jurassic 

 period (? Trias), are abundant in the Cretaceous, and have con- 

 tinued through the Tertiary period to the present day. 



Fig. 564. — Piicatula placunea. Lower Greensancl. 



The genus Piicatula (fig. 564) approaches Spondylus nearly in 

 having an inequivalve shell, which is attached by the right valve, 

 and by having two hinge-teeth in each valve. The shell, however, 

 is rarely eared, the hinge-area is obscure, and the valves are not 

 spiny, though they may be plaited. The Plicatulce extend from the 

 Trias to the present day, and they abound to such an extent in 

 parts of the Lower Greensand (Cretaceous), as to have given rise to 

 the name of " Argile a Plicatules " applied to the beds in question. 

 Lastly, the genus Terquemia, of the Triassic and Liassic rocks, re- 

 sembles Spondylus in many respects, but is at once distinguished by 

 the fact that the hinge is edentulous. In this respect the genus 

 resembles the Oysters, from which it is separated by the fact that 

 the shell is attached by the right valve, and not by the left. Fischer 

 also includes in this family, with doubt, the Carboniferous genus 

 Pachypteria. 



Family 2. Limid.^e. — The shell in this family is auriculate, and 

 equivalve or nearly so, and it may be free or may be attached by 

 a byssus, which passes out by a sinuosity in the right valve or by a 

 notch below the anterior ears. The beaks are pointed and straight, 

 and a portion of the hinge-area and ligament is left externally visible ; 

 while hinge-teeth may be wanting or present. The earliest types of 

 the family appear in the Carboniferous rocks ; there are numerous 

 Secondary and Tertiary forms ; and the present seas contain a 

 limited number of existing species. 



