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ON A RARE VARIATION IN THE SHELL OF 

 PTEROCERA LAMBIS, LINN. 



By Arthur Willey, D.Sc. 



(Communicated by Jas. P. Hill, P. L.S.J 



(Plate XIII.) 



With the vie\v of ascertaining the nature of the variations 

 which the shell of this common tropical species presented, I 

 recently made a collection, amounting to 67 specimens, both from 

 New Britain and from the Eastern Archipelago of New Guinea, 

 the majority coming from the latter locality. 



As might be expected from such a comparatively large series, 

 variations of greater or less intensity were very numerous. I am 

 indebted to Mr. Charles Iledley for his kind assistance in 

 arranging and classifying the collection. 



As is known, Bateson (Materials for the Study of V'ariation, 

 London, LS94) has divided variations into two main categories, 

 namely, (1) Meristic variations, comprising numerical variations 

 in members of a series, as the rings of an earthworm or, what 

 concerns us at present, the digitations of Pterocera, and (2) Sub- 

 stantive variations, comprising variations in the form and bulk 

 ("substance") of individual parts or regions. 



My collection shows numerous substantive variations, the more 

 striking of which relate to the curvature of the digitations, their 

 lengths, the intervals between them, and to the extent to which 

 the apical whorls of the shell are involved in, concealed by or 

 fused with the posterior digitation. The last point is essentially 

 co-terminous with the extent of the ascent of the last whorl 

 upon the spire. 



Excluding about 15 of the shells as being young, i.e., with 

 unthickened outer lips, in the majority of the adult shells a 

 greater or less number of the apical whorls are free. In two 

 specimens only, that is to say in about 1 per cent, of the' 



