48 THE LIFE OF THE MOLLUSCA 



obtained, and the first evidence of the highly 

 specialized Bivalve (Lithophagus) that burrows into 

 rock, shell, or coral. 



At the close of the Palaeozoic period many of the 

 older genera of Bivalves disappeared, but at the 

 opening of the Mesozoic period in the Trias, a 

 number of others came in. Among them further 

 representatives of the freshwater Mussels (Unio), 

 of the Thorny Oyster family (Spondylidse), and the 

 Cockles (Cardiidse). The oldest examples (Campylo- 

 sepia, Aulacoceras, and Atractites) of the group of 

 ten-armed Cuttlefish made their appearance also at 

 this time. 



The Jurassic strata are rich in Molluscan remains, 

 which sometimes form whole masses of rock. The 

 Rhipidoglossate Gastropods attained their acme of 

 development at that period, while Pectinibranchs 

 multiplied in great variety, and representatives of 

 the higher forms began to appear. Further remains 

 of Land Shells (Helix, etc.) and the earliest examples 

 of freshwater Gastropods (Planorbis, Valvata, and 

 Melania) have been recorded from the lowest Jurassic 

 beds, but there is some doubt as to the exact 

 determination of these genera in most cases. In 

 the uppermost beds, however, the well-known Purbeck 

 marble is composed of masses of Valvata, Vivi- 

 para, etc. 



Among Bivalves genuine Ark Shells (Area), 

 Anomia, and various families of Eulamellibranchs, 



