WATSON : AFFINITIES OF PYEAJIIDULA, ETC. 29 



Vallonia excentrica is common amongst grass, etc., in very many 

 parts of England. It differs from V. pulchella in having a smoother 

 and slightly smaller shell, somewhat oval in outline, with more 

 rapidly increasing whorls. The suture is shallower, and the umbihcus 

 is usually narrower in the centre, but shows more of the penultimate 

 whorl near the aperture. The peristome is gradually expanded, 

 mstead of being abruptly reflected at the edge, as in F. pulchella 

 (compare text-figs. 6e and 6A). This marked difference in the 

 peristome seems to be quite constant, and forms one of- the easiest 

 means of separating the two species, as was first pointed out to me 

 several years ago by Mr. G. H. Clapp. 



The radular teeth of F. excentrica are, on the whole, very similar 

 to those of F. pulchella, but the outer marginals are even more 

 elongated transversely, and often have more cusps. The number of 

 teeth in each transverse row is about the same as in F. pulchella, 

 but the average number of rows is slightly greater than in either 

 of the other species, notwithstanding the smaller size of the shell : 

 83 rows are the most that I have counted in F. excentrica, 77 in 

 F. costata, and 76 in F. pulchella. The jaw of Vallonia excentrica 

 also appears to differ slightly from those of the other two species, 

 being usually a little broader, with a tendency to form a slight 

 median projection, and showing some divergent striae towards its 

 lower edge (pi. I, fig. 6). 



While this species is undoubtedly very closely allied to F. pulchella, 

 it seems on the whole to be more highly specialized, and it should 

 therefore be placed last among the British members of the genus. 



Mutual Affinities of the British Species of Acanthinula. — 

 While the three British species of Vallonia are closely related to one 

 another, the very reverse is true in the case of the two British 

 species of Acanthinula. A. lamellata differs externally from A. 

 aculeata in having a globosely pyramidal shell, with narrow whorls, 

 and a simple, unreflected peristome, and in the protoconch being 

 microscopically punctate instead of spirally striate, as in ^. aculeata. 

 Internally the differences are equally great. The recurrent ureter 

 of A. aculeata is represented by an open groove, the lateral teeth 

 of the radula have an additional small cusp between the mesocone 

 and the ectocone, and the central tooth is only a little smaller than 

 the laterals ; while the penis, epiphallus, etc., are not known to 

 occur. In view of these important differences there can be no 

 doubt that Westerlund was right in placing A. lamellata in a distinct 

 sub-genus, which he named Spermodea.^ 



Summary of chief Coissclvsions.— Acanthinula and Vallonia are 

 rather closely allied genera ; Patulastra and Pyramidula (s.s.) are 

 each a little more isolated ; but all these genera are nearly related to 

 the Pupillidse, and should probably be placed in that family. They 



1 Rada Jugoslav. Akad., vol. cli, 1902, p. 90. 



