THE GASTROPODA 97 



This marginal slit, in one form or another, is character- 

 istic of an important series of gastropods, whose 

 generally primitive character is confirmed by the fact that 

 they formed a much larger proportion of the Palaeozoic 

 than of the modern gastropod fauna. 



FlG. 27. TURRITELLA IMBRICATARIA, LAMARCK, EOCENE. 



(Natural size.) (After Deshayes.) 



2. Turritella imbricataria (Fig. 27) is a very common 

 fossil in the Barton beds of the Hampshire coast, and the 

 Bracklesham beds of the Selsey peninsula. Its shape 

 approaches that of a very acute cone, but it is formed of 

 a tube coiled in a spiral, and increasing steadily in size 

 from the apex of the cone downwards, which is the 

 direction in which growth takes place. Each turn of 

 the spiral is called a whovl t and all the whorls except the 



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