230 MOLLUSCA. LAMELLIBRANCHIA 



a pallial sinus {e.g. Allorisnia) are found. Before the 

 beginning of the Mesozoic period many of the Palaeozoic 

 genera became extinct, and in the Trias a number of new 

 types appear. In the Mesozoic formations Dysodont, 

 Isodont, Schizodont (Trigoniidre), and Desmodont genera 

 are abundant, and the Heterodont forms slowly increase. 

 The Cretaceous period is particularly distinguished by the 

 abundance of Inoceramus, and by the presence of Hip- 

 purites, Radiolites and other allied genera. In the 

 Tertiary period the Heterodont group attains the greatest 

 importance. 



Fresh water lamellibranchs are generally rare in 

 the Palaeozoic and Mesozoic formations. Probably the 

 earliest form is Archanodon (= Amnigenia) juJcesi from 

 the Old Red Sandstone. In the Coal Measures several 

 species of Carhonicola, Anthracomya, and Naiadites occur. 

 The living type Unio has been found in the Inferior Oolite 

 of Yorkshire, and is fairly common in the Purbeckian and 

 Wealden of the south of England, where it is associated 

 with Cyrena, Freshwater lamellibranchs also occur in 

 the Woolwich Beds, the Oligocene deposits, and in the 

 Pleistocene river-gravels. 



The principal genera of Lamellibranchs found in the 

 different systems are as follows : 



Cambrian. Ctenodonta, Glyptarca. 



Ordovician. Ctenodonta, Cyrtodonta, Glyptarca, Modiolopsis. 



Silurian. Ctenodonta, Cardiola, Pteria, Pterinea, Ambonyehia, 

 Modiolopsis, Grammy sia. 



Devonian. Ctenodonta, Cardiola, Pterinea, Aviculopecten, Acti- 

 nopteria, Megalodon, Conocardium. 



Carboniferous. Nucula, Parallelodon, Posidonomya, Pinna, 

 Conocardium, Leiopteria, subgenera of Pecten, Aviculopecten, 



