n\' 11. LEIGIITOX KESTEVEN. 445 



Jlelo is merely an emp-hasis of the eoluniellar edge, appears on 

 the protoconch (pseudoprotoconch ?) when onh^ two whorls have 

 been formed; the third follows so soon after the second that they 

 may be said to arise simultaneously, on the completion of two and 

 one-half whorls; the fourth arises a quarter of a whorl later. The 

 last gradually degenerates after the formation of five whorls, 

 and has generall}^ disappeared b}^ the time six and one-half 

 whorls are completed, though it may occasionally persist through- 

 out the ephebic stages. 



For reasons stated in the next section of this pai:>er, the 

 structure which follows the protoconch (pseudoprotoconch 1) in 

 M. diadema is to be regarded as neanic, and it is interesting to 

 find that though there is no external differentiation, the neanic 

 stages are clearly defined, l)y the retention of the fourth fold. 



The order of succession of the plaits in M. atliiopica, Linn., is 

 doubtless on the same lines as in the above species; there are 

 generally only three plaits present in adult specimens, but one 

 example of eight whorls had all four plaits full}^ developed. 



The position of this fourth plait is always on the posterior 

 boundary of the sinus left in the track of the anterior canal. It 

 is possible that the ridge sometimes present at the posterior 

 boundary of these sinus is the cause of the occasional retention 

 of the plait. 



B. Definition of the Nepionic Stage in the Gasteropod 



Mollusc. 



Recent papers dealing with protoconchs provide a fairly wide 

 range of facts, which may be taken as the basis of speculation on 

 this interesting minor problem. 



The nepionic (brephic or silphologic) stage was defined by 

 Buckman ct Bather^ as that immediately succeeding the embry- 

 onic stages, and during which no specific characters make their 

 appearance. The next, neanic (nealogic) stage they defined as 

 that during which specific characters and all other morphological 

 features present in the adult, appear and undergo development. 



* No. 15 appended bibliography. 



