THE GASTROPODA 113 



top of another ; Calyptraa (Cret.-Rec.j, the bonnet- 

 limpet, or Chinaman's hat, is depressed-conical, with 

 a spiral shelf in the interior ; Platyceras (Camb.-Trias), 

 limpet-like, with spirally-rolled apex, often found adhering 

 to the ventral surface of crinoids, on which it was in 

 some degree parasitic. The Capididce attained their 

 acme in the Silurian and Lower Devonian, particularly 

 of Bohemia, where many strange forms occur, including 

 some which are high-conical and almost cylindrical. 



The family Melaniidte, with the turreted genera Melania 

 (Jur.-Rec.) and Melanopsis (Cret.-Rec.), is transitional 

 to the siphonostome group, the latter genus having an 

 anterior canal and the former not. Melania (Fig. 34, h) 

 is distinguished from Turritella by the shape of the whorls, 

 which gave a step-like outline to the shell, and the 

 greater strength of the vertical ornament, which develops 

 strong tubercles or spines. 



(b) Siphonostome. Cerithium has been described 

 already. Closely allied to it is Potamides, which at present 

 lives in mangrove swamps and the estuaries of tropical 

 and sub-tropical rivers. There is no absolute way of dis- 

 tinguishing this genus from Cerithium by shell-characters 

 alone, though the opercula are distinct, and in the fresh 

 shell there is a dark green periostracum (external non- 

 calcareous layer of shell, rarely preserved in fossils). 

 Consequently the reference of fossil species to Potamides 

 is partly based upon their occurrence along with un- 

 questionable fresh-water shells. Potamides is divided now 

 into several sub-genera : it is known from the Cretaceous 

 upwards, and had a much wider distribution in the 

 Eocene and Oligocene periods than now, being common 

 in the strata of the Isle of Wight. These two genera 

 are members of the family Cerithiidtt, of which other 

 members occur as far back as the Triassic period. In 

 these early genera the anterior canal is very feebly 



8 



