MORPHOLOGY OF THE HELICIKIDJE. 801 



the anterior border, cannot possibly determine the question of 

 the death or survival of the animal. The same reasoning applies 

 to the variations of the radular teeth : the function of the radula 

 is to rasp, and any of the modifications shown in figs. 60 to 65 

 is equally efficient as a vAsp. Nobody, I think, would venture 

 to assert that the minute difli'erences in the four species of 

 Aphanoconia (figs. 62 to 65) could have had any value in the 

 difierentiation of these species by natural selection. 



Asa result of my somewhat elaborately minute studies, I am 

 driven, and, I confess, somewhat unwillingly driven, to the con- 

 clusion arrived at by a number of naturalists, that natural 

 selection is efficient in preserving chaiucters of physiological 

 importance, but inefiiective in producing new species by adding 

 together numerous minute successive variations. The only 

 conclusion justified by the facts seems to me to be that the 

 characters on which systematists rightly rely are of the natvire 

 of deviations or mutations, of no consequence to the well-being 

 of the animals in which they appear, but inheritable, and there- 

 fore perpetuated under favourable circumstances by segregate 

 breeding. The Helicinidpe, inhabiting narrow areas, and often 

 segregated in remote islands, afford particularly favourable oppor- 

 tunities for segregate breeding. 



As to how far these small deviations of functionally iinimportant 

 structures may be due to the influence of external conditions I 

 do not venture to ofier an opinion, but the following fact is 

 suggestive. Among the shells in the tube containing several 

 specimens of Aphanoconia mergidensis was a, specimen which 

 in size, shape, coloration, and marking so exactly resembled the 

 others that I took it for a Helicinid (as the collector must also 

 have taken it) and decalcihed it with a view to anatomical 

 investigation. It proved to be a, Helicid, of what genus and 

 species I cannot say, as I had destroyed the shell and could not 

 find another specimen. 



Among the collection of Helicinid^e made in the Andaman 

 Islands and presented to the British Museum of Natural History 

 by Mr. G. Rogers was a tube containing half a dozen specimens 

 which differ recognizably in the characters of the shell and 

 operculum from Aphanoconia andamanica Benson, but ai'e clearly 

 closely I'elated to that species. I have not been able to refer 

 them to any named species, and as the radular characters show 

 it to be distinct from andamanica, I describe it as a new species, 

 as follows : — 



Aphanoconia rogersii, sp. n. (PI. XLII. figs. 65-69.) 



Shell oblately spheroidal, the surface marked with closely set 

 I'adial growth-lines ; colour light orange-yellow marked with more 

 or less distinct reddish-brown radial bands ; spire of 4| whorls, 

 increasing regulai-ly and somewhat rapidly in size, the last whorl 

 obtusely keeled, the keel produced into a, prominent angular 



