FOSSIL MOLLUSCA. 255 



of bivalves, Venus, commenced its existence, together 

 with Trigonia (still surviving in Australian seas), 

 . Isocardia, Tellina, Corbula, Panopcea, Bulla, Paludina, 

 etc., all of which are well-known modern forms. The 

 naturalist is as much interested, when studying the 

 life-history of the globe as revealed by geology, in 

 observing where the different kinds of families of 

 animals and plants first 

 make their appearance, 

 as a genealogist is in 

 tracing the commence- 

 ment, development, and 

 alliances of some illus- 

 trious aristocratic houses. 

 Commonest among 

 the genera of Liassic and 

 Oolitic univalve and bi- Fig " i*-**** >*" ( olite >- 

 valve mollusca are (in addition to those above-men- 

 tioned) Purpurina, Cerithium, Nerinea, Apporhais, 

 Potamides, Modiola, Ostrea, Gryphoea, Exogyra, Ger- 

 villea, Pholadomya, Anatina, Lima, Hippodium y Pecten, 

 Cucullcea, Avicttla, etc. Some of these are splendid 

 fossils ; and there are few cabinets, and perhaps no 

 museums, which do not possess Lima gigantea, Avicula 

 Cygnipes, Hippodium ponderosum, Trigonia clavellata, 

 etc. 



If the student cannot afford to travel in search of 

 specimens to the spots where they are actually found, 

 he can obtain them nowadays by various means, in 



