WATSON : AFFINITIES OF PYRAMIDULA, ETC. 13 



Pyramidula rupestris usually has one more tooth on the left side 

 of each row than on the right. Patulastra balmei, on the other hand, 

 sometimes has one more tooth on the right side than on the left. 

 In all three species of Vallonia there are often eight marginal teeth 

 on each side, instead of nine. The number of transverse rows varies 

 considerably. 



The central tooth in Pyramidula, Patulastra, Acanthinula, and 

 Vallonia is tricuspid, although the ectocones are usually very small. 

 The whole tooth is also, as a rule, smaller than the laterals : in 

 Acanthinula aculeata, Patulastra balmei, and all the species of 

 Vallo7iia, it is very much smaller and narrower than the adjacent 

 teeth ; in Acanthinula lamellata it is also somewhat smaller ; only 

 in Pyramidula is it of about the same size as the laterals. 



The lateral teeth in these genera are usually bicuspid, with 

 quadrate bases, the outer posterior corners of the bases being more 

 or less thickened, as is also the case in the central tooth. In 

 Pyramidula rupestris the mesocones of both the central and lateral 

 teeth are unusually broad, with very obtuse cusps, but this is probably 

 an adaptation to the animal's special environnjent, for it would 

 seem likely that broad rounded cusps would be best fitted for 

 scraping the surface of the hard limestone walls and rocks on which 

 this species generally lives. Helicigona lap)icida is also very 

 frequently found on limestone walls, and in this species the cusps 

 of the central and lateral teeth have undergone a parallel modification, 

 as Mr. Bowell has pointed out. In the embryonic radula of 

 Pyramidula rupestris the broadening of these cusps is not quite so 

 noticeable (text-fig. 4a), while in P. humilis (Hutton) it has not taken 

 place at all (judging from a radula in the late Professor Gwatkin's 

 collection). Excepting in P. rupestris, there is a decided gap between 

 the mesocone and the ectocone of the lateral teeth, and in Acanthinula 

 lamellata this gap is occupied by a small additional cusp, such as we 

 also find in the genus Vertigo} In Vallonia, and in the embryo of 

 Patulastra balmei, the first lateral teeth are unusually large (text- 

 figs. 4& and M). 



The marginal teeth in Pyramidula, Patulastra, Acanthinula, 

 and Vallonia are more numerous than the laterals, and are 

 characteristically pectinate, having broad bases bearing a number 

 of narrow cusps. The mesocone forms the first or innermost of these 

 cusps. The remainder are smaller, excepting in Pyramidula rupestris, 

 and are formed by the multiplication of the ectocone. No endocones 

 are present in any of these genera. 



Now, pectinate marginal teeth of this type do not occur in the 

 Endodontidae, nor in any other sigmurethrous family with which I 

 am acquainted. It is true that in small snails of various types, and 

 especially in those with narrow whorls, the outer marginal teeth" tend 



1 See Bowell, Journ. of Conch., vol. xii, 1909, pi. v. 



