THE AFFINITIES OF PYRAMIDULA, PATULASTRA. 

 ACANTHINULA, AND VALLONIA. 



By Hugh Watson, M.A. 



Read 12th December, 1919. 



Introduction. — Much uncertainty seems to prevail about the 

 true affinities of some of the smaller snails found in the British Isles. 

 Thus, Pyramidula rupestris (Drap.) and Patulastra balmei (P. & M.) ^ 

 — a species introduced into Ireland from the South of Europe — ■ 

 are commonly placed in the Endodontidse ; that is to say, in the 

 same family as Goniodiscus rotundatus (MiilL), and even, as a rule, 

 in the same genus. Vallonia and Acanihinula, on the other hand, 

 are still often placed in the Helicidse, between Hygromia and 

 Helicodonta, although it is nearly twenty years since Dr. Pilsbry 

 suggested that Vallonia should be removed from that family.^ The 

 chief purpose of the present article is to try to dispel this un- 

 certainty, and to show that Pyramidula, Patidastra, Acanihinula, 

 and Vallonia are fairly closely related to one another, but that these 

 genera have very little affinity with either the Endodontidse or the 

 Helicidae, their nearest British allies being among the forms assigned 

 to the Pupillidse, CochHcopidse, and Enidae. 



la 



C^. rotUnAatu* 

 lb 



laTnellata 

 le 



Figs. la-c. — Transverse sections through the foot of Pyramidula, Goniodiscus, 

 and Acanihinula ; showing the structure of the pedal gland, the presence 

 or absence of peripodial grooves, etc. 



The compilation of this paper has been greatly facilitated by 

 the kindness of Dr. A. E. Boycott, who has allowed me to study 

 his beautiful serial sections of Acanihinula and of several other 

 small British snails. I am much indebted to Mr. A. W. Stelfox for 

 preserved specimens of Patulastra halmei (P. & M.) and Helicodiscus 

 lineatus (Say) from Ireland, and to Mr. W. E. Alkins for some living 

 examples of Vallonia excentrica from Staffordshire. My thanks 

 are also due to Dr. Boycott and Mr. B. B. Woodward for the loan of 



I = P. flavida (Ziegler) ; see Man. Oonch. (2nd Rer.),vol. iii, 1887, p. 30. 

 Proc. Acad. Nat. Sci. Phila., 1900, p. 564. 



