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 Palzozoic 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 (Spondylidz), and the 
Cockles (Cardiidz). The oldest examples (Campylo- 
sepia, Aulacoceras, and Altractites) 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 (Arca), 
Anomia, and various families of Eulamellibranchs, 
