Notes on South African Mollusca. 133 



seems probable. Or perhaps connecting links may be found between 

 one or two of these groups and snails now placed in other families. 

 The line drawn between the Acavidae and the Helicidae is still 

 somewhat vague; and perhaps the gap which separates the Acavidae 

 from the Corillinae, Camaeninae, and Polygyrinae may be no greater 

 than that which separates these subfamilies from the more highly 

 organized Helicinae. If this is the case it is possible that just as 

 the dart-bearing Helices have probably arisen from the more primi- 

 tive Helicidae, so these may have in turn been evolved from the 

 Acavidae. On the other hand, those genera of the Helicidae in 

 which the reproductive system is most like that of the Acavidae 

 have generally been found to possess very different pallial organs ; 

 and there is some ground for believing that the Helicidae may have 

 arisen further north than the Acavidae, and that the resemblance 

 between some members of the two families may be entirely due 

 to convergent evolution. Perhaps a comparative study of the 

 structure of the pedal gland in the two families might throw 

 some light on this question. 



The affinities between the Bulimulidae and the Acavidae are a 

 little less doubtful, for the more primitive members of these two 

 families show a decided resemblance to each other. Pilsbry has 

 demonstrated that it is almost impossible to distinguish the Bulirni- 

 form Acavidae from the Bulimulidae by their shells * ; and in their 

 internal anatomy some genera of the latter family show an undoubted 

 similarity to the Acavidae. Thus, according to Pilsbry's description 

 and figures,! the genus Macrodontes is remarkably like the Acavidae 

 in its pallial organs, its jaw, its radula, its retractor muscles, and 

 in the general features of its reproductive system ; and yet Macro- 

 dontes is placed in the Bulimulidae near Odontostomus, a genus 

 which it closely resembles in- its conchological characters. The 

 theory that the Acavidae have arisen from the same stock as the 

 Bulimulidae is also supported by a comparison of the distribution 

 of the two families ; for we find that the Bulimulidae are widely 

 distributed in those regions of the world to which the primitive 

 Bulimiform Acavidae have also become restricted, but that they 

 occur nowhere else.]: While, therefore, the two families have 

 evolved along widely divergent lines, it seems not unlikely that 

 the Palaeozoic group of snails from which the Bulimnlida?, have 

 descended gave rise to the Acavidae also. 



* Man. Conch., 1902, Index to vols x.-xiv. p. vii. 

 t Man. Conch., 1901, xiv. p. 29. PI. V, f. 37, 38, PI. XV, f. 28, 29. 

 J See Rep. Princeton Univ. Exped. Patagonia, 1896-99, iii. (1911), p. 630, 

 f. 37. 



