UNIVALVE MOLLUSC A. 



437 



Littorina, periwinkles, and P fronts, as an example of this latter genus, 

 we have P. Conchyliophorus (Fig. 223, page 443). 



The Staircase-shell (Solarium perspectivum\ is recognised by its 

 deep umbilicus, wide and funnel-shaped, in the interior of which may 

 be seen the little crenated teeth which follow the edge of every turn 

 of the spiral up to the top. In most collections of these pretty shells 

 we find this Staircase-shell (Solarium perspectivum'} of Lamarck, from 

 the Indian Ocean (Figs. 235, 236), the diameter of which is sometimes 

 two inches and a half. The Australian Sun-dial (S. variegatum, 

 Linnaeus, Fig. 237) is another species frequently seen in collections ; 

 it is as much variegated above as below, of a white and rusty brown 



Fig. 236. Solarium pcrsfcctivum. 



Fig. 235. Solarium perspectivum. 



Fig. 237. Solarium vanegatum. 



Another species, the minute, trellised Sun-dial, which is only ten 

 lines in diameter, comes from the coast of Tranquebar. 



The eleventh family, Turritellidce, has, as typical genera, Vermetus, 

 Scalaria, and Turritella, which last is a genus with a great number 

 of species, many of which are found in every sea. All these shells, 

 as their name indicates, represent a winding pyramid, terminating in 

 a sharp point, some of them having fluted spirals, others rounded, 

 angular, or flat, and some of them elegantly pencilled. Figs. 238 to 

 242 represent some of the species of the genus. 



The twelfth family, Melaniada, contains the fresh-water genera, 

 Paludomus and Melania. 



The thirteenth family, Cerithiada, contains the genera Potamides, 

 Aporrhais, and Cerithium. 



The species of the genus Cerithium are marine, and are chiefly 

 found in muddy bottoms, and more frequently at the mouths of rivers. 



