WATSON : AFFINITIES OF PYRAMIDULA, ETC. 7 



reprints of three foreign papers which I was unable to consult in 

 Cambridge. 



Evidence of the Foot. — A mere examination of the outside of 

 the foot of Pyramidula rupestris and of Patulastra halmei is enough 

 to show that these species cannot be closely allied to Goniodiscus 

 rotundatus or Helicodiscus lineatus, or, indeed, to any form rightly 

 assigned to the family Endodontidee as defined by Pilsbry ; ^ for 

 both these species resemble Acanihinula and Vallonia in having no 

 peripodial grooves. The striking nature of this difference between 

 Goniodiscus on the one hand, and Pyramidida and Acanihinula on 

 the other, may be seen from text-fig. 1. This figure also shows that 

 Pyramidida rujjestris resembles Acanihinula much more closely than 

 Goniodiscus in the structure of its pedal gland, and the same is true 

 of Vallonia. Further, the type of pedal gland that is found in 

 Acanihinula, Vallonia, and Pyramidula occurs also in the Pupillidse 

 and the Cochlicopidse, and these families are also without peripodial 

 grooves. We see, therefore, that the evidence of the foot strongly 

 supports the view that these three genera are all more nearly related 

 to the Pupilhdas and the Cochlicopidse than to the Endodontidse. 



Evidence op the Size. — The HeHcidse, however, are also without 

 peripodial grooves. But in this family the dorsal wall of the duct 

 of the pedal gland is longitudinally folded,^ which is not the case 

 in Acanihinula, Vallonia, or Pyramidula. Moreover, the very small 

 size of these snails at least suggests that they may not be rightly 

 assigned to the Helicidse. It is true that the Rev. E. W. Bowell has 

 expressed the opinion that size " has counted for too much in our 

 systems of classification ".^ But he goes on to point out that an 

 increase or diminution of size in an organism necessitates a re- 

 distribution of symmetry, because the constituent cells do not change 

 their size proportionately, and that this rearrangement often 

 involves a considerable morphological change. It would therefore 

 seem improbable that a very great alteration in size could be easily 

 and quickly effected in the course of evolution ; and, if this be the 

 case, the species of Vallonia and Acanihinula are not likely to be 

 very closely related to Helix pomaiia and its allies. 



Evidence op the Central Nervous System. — Most students of 

 comparative anatomy, however, would attach more weight to 

 evidence afforded by the central nervous system than to mere 

 considerations of size. Now the central nervous system in the 

 Helicidse is characterized not only by the close aggregation of 

 the pedal, pleural, and visceral ganglia, but by the fact that the 

 abdominal ganglion is completely united with the left parietal 

 ganglion (see text-fig. 2c). Very different is the central nervous 

 system of Vallonia, Acanihinula, Paiulasira, and Pyramidula. In these 



1 Man. Conch. (2nd ser.). vol. ix, 1894, p. xxviii. 



2 Andre, Rev. Suisse de Zool., vol. ii, 1894, p. 298, pi. xii, fig. 5. 



3 Proc. Make. Soc. Lond., vol. viii, 1909, p. 379. 



