TEETH OP ACHATINELLID^E. XI 



the cusps, it is not necessarily of inueh systematic importance. 

 The amount of change in various groups is apparently some 

 indication of the length of time since arboreal habits were 

 assumed. In Achatinellidce and Tornatellinida? the modifica- 

 tion of the teeth is wholly unlike that usual in arboreal 

 snails of other families, in which the cusps are always broad- 

 ened. The Achatinellid type is really a further development 

 of the multicuspid teeth of small ground snails, and it would 

 seem, not an adaptation induced by arboreal life, though it 

 has obviously proved efficient in that station, in the absence 

 of any competitors. 



The multicuspid cutting edges of the teeth, and the absence 

 of differentiation into laterals and marginals, cause me to 

 view the Achatinellid radula as one from which central and 

 lateral teeth have been eliminated, leaving only marginals. 

 The marginal teeth of Amastridtz and Enida, as of many 

 other snails, stand in somewhat oblique transverse series. If 

 the median field was eliminated, the transverse rows would 

 be broadly V-shaped. Pachnodus in the Enida, as figured 

 by F. Wiegmann, has somewhat such a radula as that we may 

 suppose the ancestors of Achatinella had. The teeth of 

 Pachnodus are very numerous, 375 to 393 in a row, the mar- 

 ginal fields have the rows strongly oblique, and the marginal 

 teeth are multicuspid. The teeth of Achatinella differ from 

 the marginals of Pachnodus, or of Leptachatina, chiefly by 

 the long and narrow basal-plate, whereas in most marginal 

 teeth this plate is very short and broad. 



By a similar reduction, some species of Mesomphix have 

 practically eliminated the lateral teeth, while others retain a 

 few of them. The closely related Omphalina has the usual 

 development of laterals. The Agnatha and Agnathomorph 

 snails also have lost all laterals. 



I believe, therefore, that all the teeth of Achatinellid^ have 

 been derived from the marginals of some unspecialized group 

 of ground snails having multicuspid marginal teeth. This 

 ancestral group was evidently also ancestral to the Amastrida, 

 in which Leptachatina still retains multicuspid marginal 

 teeth; but even the most primitive existing Amastrida? have 



