22 PROCEEDINGS OF THE MALACOLOGICAL SOCIETY. 



Steenberg has shown/ it is not a primitive member of the Clausiliidse, 

 but a highly specialized form, allied to Clausilia bihlicata. The 

 only reasonable explanation of the characters of the shell of Balea 

 perversa seems to be that this species is a Clausilia which has 

 sacrificed the completion of its own shell in its efforts to provide 

 adequate shells for its young. And it seems likely that the same 

 explanation applies to Pyramidula. ■■ For in the bleak, rocky 

 situations in which Pyramidula rupestris is so often found, it is 

 obviously specially advisable that the young should come into the 

 world adequately protected. 



If Pyramidula is simply a kind of Pupilla that never grows up, 

 it clearly must be placed in the Pupillidse. But we have already 

 seen that the genus Pyramidula does not differ much from 

 Acanihinula and Vallonia, excepting for the larger central teeth 

 of the radula. In Acanthinula lamellata, however, the central teeth 

 are not very much smaller than the laterals, and they are no smaller 

 in A. (Zoogenites) harpa, according to Morse.^ This feature, there- 

 fore, cannot be said to separate the Valloniidse from the Pupillidae, 

 and there seem to be no other anatomical differences. The Helici- 

 form shell of the Valloniidse is not an important difference, for, 

 according to Pilsbry, more than half of the sub-families into which 

 he divides the Pupillidae contain Helicoid forms. ^ And although it 

 is easy to attach too much weight to the " recapitulation theory ", 

 the fact that so many of the Pupillidae are Heliciform when young, 

 even though they are not when full-grown, suggests the possibility 

 that the spire of the ancestral form of the family may have been 

 no higher than that of Acanthinula, for example. - There is some 

 reason to suppose that the Pupiform members of the Streptaxidae 

 may have been evolved from the Helicoid forms, and possibly the 

 course of evolution in the Pupillidae may have followed parallel 

 lines. Moreover, certain recent authors have already placed 

 Acanthinula in the Pupilhdae,^ and if Acanthinula should be assigned 

 to that family, so should Vallonia. In other words, the Valloniidae 

 should be reduced to the rank of a sub-family of the Pupillidae, like 

 the Vertigininae, etc. Patulastra should probably be placed in the 

 same sub-family as Vallonia and Acanthinula ; though possibly it 

 would be better placed in a separate sub-family of the Pupillidae, on 

 account of its very different reproductive system. 



But Vallonia, Acanthinula, and Patulastra seem also to be very 

 closely related to the Enidae. This is due to the fact that the 

 Palaearctic Enidae do not differ in any essential features from the 



1 Anatomie des Clausilies Danoises : Mindeskrift for J. Steenstrup, ' No. 29, 

 1914, pp. 39, 40, 43. 



2 Binney, Terrest. Air-breathing MollusJcs of the U.S., vol. v, 1878, p. 341, 

 fig. 225. 



2 Man. Conch. (2nd ser.), vol. xxiv, 1918, p. x. 



4 e.g. Pilsbry, ibid, (same page) ; C. R. Boettger : Nachr. Deutsch. 

 Malak. Gesell., vol. xli, 1909, p. 4 ; vol. xliii, 1911, p. 24, 



