700 



LAMELLIBRANCHIATA. 



In the genus Lima or Radula the shell is equivalve and free, and 

 the beaks are separated from one another and eared (fig. 565). The 



surface is generally adorned with 

 radiating ribs or striae, and there is a 

 median cartilage-pit, and a triangular 

 hinge-area. The genus (including 

 Plagiostoma) appears to occur in the 

 Carboniferous, is abundantly repre- 

 sented in the Triassic, Jurassic, 

 Cretaceous, and Tertiary rocks, and 

 exists in diminished numbers at the 

 present day. Limea, ranging from 

 the Trias to the Recent period, 

 differs from Lima principally in hav- 

 ing the hinge-line closely toothed. 



Family 3. Pectinid^. — In this 

 family the foot is tongue-shaped and 

 byssiferous ; the margins of the 

 mantle-lobes carry ocelli ; and the 

 adductor impression is a little ex- 

 centric. The shell is sub-equivalve or equivalve, nearly equilateral, 

 auriculate, free or occasionally attached by the right valve. Gener- 

 ally there is a notch under the ear of the right valve for the passage 



Fig. 565. — Lima {Plagiostoma) gigantea. 

 Lias. 



Fig. 566. — Pecten Islandicus, left valve. Post-Tertiary and Recent. 



of the byssus ; and the cartilage-pit is internal, of triangular shape, 

 and placed between the beaks, being rarely broken up into detached 

 pits. All the members of this family are marine, its earliest forms 



