WATSON : AFFINITIES OF PYEAMlDULA, ETC. 27 



probably an orthurethrous genus allied to Vallonia and Acanihinula, 

 judging from Hesse's preliminary description of A. triaria, Fr./ 

 and it has recently been placed in the Pupillidae by Pilsbry.^ It must 

 not be supposed, however, that this is likely to be the case with many 

 of the numerous small snails, chiefly found in the Southern Hemi- 

 sphere, which Pilsbry placed in the Endodontidse. For although the 

 shells in some of these forms are very like Patulastra or Acanihinula, 

 it is certain that in the great majority of cases this resemblance is 

 purely superficial. 



Mutual Affinities of the Beitish Species of Vallonia. — The 

 three forms of Vallonia that live in the British Isles are closely 

 related to one another, and many collectors doubt whether they are 

 specifically distinct.^ Nevertheless, I think that Dr. Sterki was 

 certainly right in regarding them as distinct species,"* for each is 

 distributed over a very wide area in Europe and America, they are 

 sometimes found together, and yet they do not appear to merge 

 into one another, but differ constantly in several characters. Perhaps 

 the failure of many collectors to appreciate the specific differences 

 is due partly to the minute size of these snails, but chiefly to the 

 fact that comparative descriptions and figures of the three species 

 have hitherto not been very accessible to English students. 



Vallonia costata is probably the most primitive of the three 

 species, and should be placed first. It differs from the others not 

 only in being furnished with conspicuous periostracal ribs, and in 

 having more distinct microscopical spiral striae on its protoconch, 

 but also in the general form of the shell, and particularly in the 

 deflection of the aperture (see text-fig. 6a). This last feature makes 

 it easy to distinguish fossil specimens of this species, however worn 

 they may be. 



The radula of Vallonia costata differs from those of the other two 

 British species in that the lateral teeth are five in number instead 

 of four, the first being not quite so large as in V. pulchella, and 

 their basal plates are more nearly square. Moreover, the marginal 

 teeth usually have about five cusps, instead of six to eight, as in 

 the other species. 



This is perhaps the commonest species of Vallonia in England. 

 It occurs with both the other species amongst grass, moss, etc., 

 and also in drier situations, such as amongst ivy on the tops of walls, 

 where it is frequently associated with Lauria cylindracea. 



Vallonia pulchella is rather more local in its distribution, and 

 seems to occur more often in damp situations. It appears to have 



1 Nachr. Deutsch. Malak. GeselL, vol. xlvii, 1915, p. 58. 



" Op. cit., vol. xxiv, 1918, p. x. 



^ e.g., Cooper, Journ. of Conch., vol. xi, 1906, p. 340 ; Adams, ibid., p. 364. 



■* See his excellent " Observations on Vallonia " : Proc. Acad. Nat. Sci. 

 Phila., 1893, pp. 234^79, pi. viii ; as well as his shorter account ot the genus 

 in Man. Conch. (2nd series), vol. xiii, 1893, pp. 247-61, pis. xxxii, xxxiii. 



