202 HALF AN HOUR WITH SHELL-FISH. 



and teeth of the Purpura furnish good microscopic 



objects. 



Underneath the stones and in the rock-pools there 



occur in abundance a minute shell, gracefully spiral, 

 called Rissoa, of which there are several 

 10 * species, one of which is to be met with 

 in brackish water. The Naticas are 

 much larger shells, one species (N. 

 monilifera, Fig. 110) exceeding the 

 periwinkle in size. The surface of its 

 whorls is mottled with pretty chocolate 



Natica monilifera. . f J 



markings, but when alive the shell is 

 Fig. ill. almost entirely covered with the mantle. 

 Cerithium resembles the Turritella in 

 its elongated form, but is much more 

 prettily ornamented with wavy, ver- 

 tical ridges. Both these shells are 

 very common on all our British shores. 

 Not unfrequently there may be seen a 

 little shell, whose name of " Tusk," or 

 " tooth-shell," easily leads to its iden- 

 tification, for it exactly resembles a 

 miniature tusk of an elephant, or the 

 canine teeth of a rodent. This is the 



Dentalmm. 



Dentalium, Fig. Ill, and it belongs to 

 the gasteropods, having all the principal features of 

 that family. The Coeeum is a shell allied to it, which 

 seems as if the Dentalium had been broken in 

 halves, and the end plugged up. When empty, the 

 Dentalium is seen to be open at each end. 



