150 MOLLUSCA. LAMELLIBRANCHIATA. 



Trias. Ostrea, Lima, Pecten, Gervillia, Area, Nucula, Myoplioria, 

 Cardita, Cardium. 



Jurassic. Ostrea, Grypheea, Lima, Pecten, Avicula, Gervillia, Tri- 

 gonia, Modiola, Hippopodium, Pinna, Area, Nucula, Cardinia, Myacites, 

 Astarte, Cardium, Pholadomya, Thracia. 



Cretaceous. Ostrea, Exogyra, Spondylus, Lima, Pecten, Gervillia, 

 Inoceramus, Perna, Tngonia, Hippurites, Radiolites, Cardium. 



Eocene. Pecten, Pectunculus, Astarte, Cardita, Crassatella, Cliama, 

 Lucina, Cardium, Cyprina, Cytherea, Panopcea, Pholadomya. 



Oligocene. Ostrea, Mytilus, Cyrena, Cytherea, Psammobia, Corbula. 



Pliocene. Pecten, Mytilus, Pectunculus, Nucula, Astarte, Lucina, 

 Cardium, Cyprina, Venus, Artemis, Tellina, Hya. 



CLASS. GASTEROPODA. 



The Gasteropoda contains even a larger number of 

 animals than the preceding class ; well-known examples 

 are the snail, the whelk and the cowry. The bilateral 

 symmetry, so characteristic of the lamellibranchs, is 

 generally obliterated, owing to the twisting of the 

 visceral-sac and the atrophy of some of the organs on one 

 side of the body. There is a distinct head, which bears 

 tentacles, and usually eyes. On the ventral surface of the 

 body is a large fleshy foot ; this is usually sole-like and 

 used for crawling, but in the Heteropods it is in the form 

 of a flattened fin, in the Pteropods it is wing-like. The 

 mantle is never divided into two lobes. Respiration 

 takes place by means of a pulmonary chamber, or by gills; 

 the latter are placed in a sac formed by the mantle ; 

 sometimes they are present on both sides of the body, but 

 very often those on the left have disappeared. In some 

 forms the mantle at the opening of the gill-sac is 

 produced into a tube, known as the siphon, by means of 

 which water passes to the gills. The mouth is at the 

 anterior end of the body; the anus is occasionally 



