LAMELLIBRANCHIATA. 



227 



shaped ligamental area (fig. 183). The teeth are numerous 



and transverse, and the surface is generally strongly ribbed. 



Species of Area have been 



described from the Lower 



Silurian Rocks upwards. It 



is probable, however, that 



the older Palaeozoic forms 



referred here really belong 



to other genera, especially 



Ctenodonta and Cyrtodonta. 



In Cucullcea the shell is 

 ventricose, and the hinge- 

 teeth are few and oblique, 

 and at each end of the hinge 

 become parallel with the 

 hinge-line. Species of this genus have been described from 

 the Lower Silurian upwards. 



The Pectunculi have a nearly round and equilateral shell, the 

 beaks separated by a striated ligamental area, the hinge-line 

 curved, and the hinge-teeth forming a semicircular row. Pect- 

 wiculus is a comparatively modern genus, and does not seem 

 to have come into existence before the Cretaceous period. 

 Numerous species are known in the Tertiary Rocks. 



The NucultK (fig. 184) have a trigonal shell, the beaks of 

 which are reversed, and turned towards the posterior side of 

 the shell, which is also the shortest side. The hinge has 



Fig. 183. Area antiqua. 



Fig. IBS. Ctenodonta contracta. 

 Lower Silurian, a Interior of right 

 Fig. 184. unciila bimrgata. Gault. valve; b Exterior of the same. 



numerous teeth on each side of a central internal cartilage-pit. 

 The Palaeozoic shells referred to Nucula probably belong to 

 other genera. Many species, however, are known from the 

 Secondary and Tertiary Rocks. 



The genera Ctenodonta and Cucullella are both nearly re- 

 lated to one another and to Nucula, and both are exclusively 

 Palasozoic, and are mainly, if not entirely, Silurian. The 

 Ctenodonta (fig. 185) are extremely like Nucula, but are dis- 



