Quarterly Joiinial of Coiicholo^y. 345 



animal. In Z. //iiiltidcnfaii/s, rows of teeth appear at an early age, 

 and as often as the shell grows a quarter of a whorl, a new row is 

 produced while the earliest is worn away. So the shell grows to 

 maturity, always having three or four rows of denticles. In this 

 variety oifuivus, however, this process seems to cease long before 

 the shell reaches maturity, and the last whorl is thus left without 

 teeth. 



The cause of these denticles can hardly be decided, but one 

 may guess. I found a small, white, tender grub, which lives in 

 beds of leaves and preys on small snails like Zonites arboreits Say., 

 by entering the shell at its mouth and devouring the animal. The 

 denticles may have been evolved as a protection against foes of 

 this description, either by obstructing their entrance, or perhaps 

 by wounding their bodies, which seems possible in the case of 

 Z. nuiltidcutatiis and significans Bland, and Strohila labyriiithica 

 Say. The complicated lip apparatus of some of our Helices (as 

 H. auriciilata Say.) and Papas (as P. contract a Say.) possessed only 

 by tlie mature shells seems likewise a defence against external 

 enemies. In the gastrodont snails the growth of the shell seems 

 slow, and the mature state short, as shown by the greater 

 frequency of young shells. Indeed, this and the fact that speci- 

 mens of Z. fiilviis in my possession laid eggs when wanting a 

 whole whorl of maturity, leads one to believe that the last whorl, 

 like white hair in man, is attained only in old age, and that teeth 

 or folds at the completed peristome would thus protect compara- 

 tively few individuals. Under these circumstances it is not strange 

 that internal denticles, present at all stages of growth, should have 

 been evolved for the jDrotection of this curious variety, which 

 offers a fair example of a species in the course of development 

 but not yet crystallized to a firm type, by the extinction of inter- 

 mediate forms. 

 Jan. 20th, 1 8 78. 



