482 



LAMELLIBRANCHIATA. 



ant and widely-distributed genus Inoceramus (figs. 341, 342), 

 which is entirely confined to the Secondary period, and is 

 mainly characteristic of the Cretaceous series. The shells of 



Fig. 341. Inoceramus sulcatus. Gault (Cretaceous;. 



this genus are inequivalve, with radiating ribs or concentric 

 furrows, and with prominent beaks. The hinge-line is long 

 and straight, with numerous cartilage -pits. Some of the 



Fig. 342. Inoceramus proNematicus. Chalk. 



Inocemmi attain a length of two or three feet, and fragments 

 of them are often found perforated by boring sponges. 



The last group of the Avicididce, sometimes raised to the 

 rank of a separate family, is that of the Pinnidce, by which 



Fig. 343. Pitma sp., interior of a broken right valve, showing the hinge. 

 Carboniferous Limestone. (Original.) 



we are naturally conducted to the true Mussels (Mytilidce). 

 In Pinna (fig. 343), the type -genus of this group, the shell 



