BY ARTHUR AVILLEY. 1 1 1 



individuals, was the apex of the spire entirely fused with and, in 

 one of them, deeply imbedded in the base of the posterior 

 digitation. In the other shell the apex was not imbedded in the 

 posterior digitation, but was applied very closely against it. 



Fterocera also varies very much as to the stage of growth at 

 which the deposition of callus on the outer lip of the shell takes 

 place. As is known, this deposition of callus eventually leads to 

 the complete closing up of the canals which, in the younger shells, 

 passed from the mouth of the shell into the tubular digitations. 

 This fact is analogous to what has been observed in some other of 

 the lower animals, namely, that they can become sexually mature 

 at very different sizes, and then cease to grow in linear dimensions. 



In the adult animal of P. lamhis, therefore, the border of the 

 mantle is not digitated. 



We now pass on to the description of the rare variation 

 referred to in the title of this paper. 



Out of the whole collection only three specimens exhibited a 

 variation in regard to the number of the labial digitations. 

 In all cases the intercalated digitation occurred lietween the 

 second and third normal digitations. Althougii small, its presence 

 offered a striking contrast to the other shells. Of the three 

 specimens exhibiting this variation, two (Figs. 1 & 2) came from 

 New Britain. In both cases the rudimentary digitation was 

 l>acked up by a definite ridge on the outer surface of the shell as 

 is the case with normal digitations. 



The third specimen, from New Guinea (Fig. 3), presented a 

 rather puzzling aspect. The intercalated digitation had a doul)le 

 character, and was not backed up by a prominent ridge on the 

 outer surface. It appeared to have had a distinctly later origin 

 than in the other two cases. Two furrows proceeded from it to 

 the mouth of the shell, one being independent and the other 

 produced by a bifurcation of the furrow belonging to the second 

 normal digitation. 



The constancy in the position of the above described rudi- 

 mentary intercalated digitation in P. lamhis should be emphasized. 



