MESOZOIC FAUNAS 189 



Aequipecten and Chlamys were especially important. 

 Spondylidae, probably derived from the Pectinidae, and 

 showing phylogerontic tendencies towards sessility, were 

 represented by Plicatula and Spondylus, both genera 

 being very common in the Cretaceous. Limidae are 

 very uniformly distributed through the periods. Plagio- 

 stoma is common in the Lower Lias and again in the 

 Chalk. Limatula and Ctenostreon are essentially Oolitic. 

 The Dysodont genera Mytilus and Modiolus (PL xiv. 

 fig- 3)> both still living, were represented throughout the 

 Mesozoic, the latter being especially abundant in the 

 Rhaetic and Lower Jurassic. Hippopodium, a well- 

 known Liassic type, is probably akin to the " Mussels," 

 and seems continuous with the Silurian Modiolopsis-stnts. 



The small order Anomalodesmacea was more fully 

 developed in Mesozoic than later periods. Pleuromya 

 and Gresslya were two abundant genera of the Pleuro- 

 myacidae, a family restricted to the era. The former 

 genus is especially common in the Inferior Oolite and 

 Portlandian. Pholadomya and Goniomya similarly 

 represented another family, but the former has one 

 species still living. It is especially abundant in the 

 Lower Lias and Inferior Oolite. 



Among Teleodesmacea, all types of hinge-structure 

 appeared, although the most abundant Mesozoic forms 

 had the relatively simple " Diogenodont " dentition. 

 Trapezium and Arctica (Cyprina} represented the Cypri- 

 cardiacea ; Astarte, Crassinella and Opis are common 

 Astartacea of the Oolites (the first-named being more 

 important in the , Cainozoic) ; while Cyrena (abundant 

 in the Wealden), Cardita and Diceras (especially 

 Neocomian) introduced their respective super-families. 

 The most extraordinary series of Pelecypoda known, 

 the Rudistacea, were probably derived from the last- 

 named type. They were massive, sessile forms re- 



