706 



LAMELLIBRANCHIATA. 



radiating ribs or concentric furrows, and with prominent beaks. 

 The shell is earless, and the hinge-line is long and straight, with 

 numerous cartilage-pits. Some of the Inocerami attain a length of 



Fig. 571. — Inoceramus sulcatus. Gault (Cretaceous). 



two or three feet, and fragments of them are often found perforated 

 by boring sponges. The shell in the Inocerami consists of a thin 

 internal nacreous layer, with a generally very thick outer prismatic 



Fig. 572.— Inoceramus firoblematicus. Chalk. 



stratum, but either the internal or the external layer may be destroyed 

 in the process of fossilisation. 



The Jurassic and Cretaceous genus Aticella (fig. 573) is related to the 

 preceding, but may be considered as the type of a separate group of the 



Aviculidce. In this genus, the shell (fig. 

 573) is very inequivalve, the left valve 

 being convex and the right valve flat, and 

 the beaks of both valves being inflated 

 and incurved. The ears are imperfectly 

 developed ; the surface is concentrically 

 striated ; the ligament is external and 

 linear ; and the hinge is hardly or not at 

 all toothed. The Carboniferous genera, 

 Rutotia, Posidoniella, and Aphanaia, and 

 the Triassic genus Rhynchopterus, are 

 placed by Fischer in the same group with Aucella. 



Fig. 573. — Aucella mosqucnsis 

 Upper Jurassic, Russia. (After Zit 

 tel.) 



The genus Moiiotis is the type of another group of the Aviculida, 

 in which the shell is equivalve, compressed, with small beaks, the 

 ears more or less confluent with the shell, the hinge without teeth, 

 and the ligament linear. Monotis itself is confined to the Trias, 

 occurring in rocks of this age in both the Old World and the New. 

 Very closely related to the preceding are the Triassic genera Dao- 



