52 UNIVALVES. 



has its two lips, inner and outer ; the outer one being often toothed, 

 the inner being occasionally so. At the base is the canal, which in 

 our example is very short and abrupt, but which in some genera is 

 of considerable length, in others represented by a mere notch, and 

 in many absent altogether. This is the anterior canal; in a few- 

 cases there is another canal, the posterior one, which is at the 

 opposite side of the mouth. 



The mouth has many varieties a few of them are shown in our 

 illustration and we will use it as our chief aid in identification, 

 beginning with those shells in which it has neither notch nor canal. 

 There are a few forms it will simplify matters to get out of the way 

 at the outset. For instance, there are two tubular shells, Dentalium 

 and Cczcum. Dentalium and its allies we have already dealt with. 

 Ccecum, which is a very small affair, is like a section of Dentalium 



Suture .'"- 



Body whorl 



"*" Outer lip 



Canal 



PARTS OF A UNIVALVE SHELL. (Nassa reticulata.) 



when fully grown, for it loses its spire very early in life, and thus 

 reduces itself to a straight or slightly curved cylinder. 



One genus, Natica, has a shell that is so markedly globular as to 

 be at once distinguishable from the others. Four genera have shells 

 that are ear-shaped. Of these, Haliotis can be sorted out at once 

 from its large size and from its perforated lip. Through these per- 

 forations pass the tentacular appendages of the mantle, and as the 

 animal grows the early holes are successively obliterated. Haliotis 

 is represented in the British list by the one species tuberculata, which 

 is frequently described as purely a Channel Islands native, but the 

 specimen figured in our coloured plate came from South Devon. 



