WATSON : AFFINITIES OF PYKAMIDULA, ETC. 19 



lamellata and A. aculeata, and depicted a couple of curved darts as 

 belonging to the latter species, although he does not mention them 

 in the text.^ In 1884 Ashford described and figured a dart and dart- 

 sac in Vallonia pulchella, stating that the dart was straight, acutely 

 conical, and 0"2 mm. in length.^ Ashford, however, said that his 

 information concerning these organs was offered subject to con- 

 firmation or correction, as further examination was desirable ; and 

 Steenberg has recently denied the existence of a dart-sac and dart 

 in Vallonia and Acanthinvla.^ 



In all the examples of these snails that I have examined, I have 

 never found a dart ; yet I would hesitate to say positively that 

 one is never developed, and that all the older authors were quite 

 wrong. It seems extremely improbable that the vagina of Vallonia 

 or Acanthinula could ever develop a dart-sac, but there is much 

 to be said in favour of the view that the enlarged basal portion of 

 the penial appendix of the Orthurethra is homologous with the dart- 

 sac of the Sigmurethra.* The distance between the proximal end 

 of the penial appendix and the genital atrium varies in the 

 Orthurethra, and in some of the Bnidae the appendix seems to 

 occupy exactly the position that the dart-sac holds in Zonitoides 

 and many other members of the Zonitidse.^ Moreover, the dart-sac 

 in the Zonitidae often has no dart, and sometimes it may bear a 

 long continuation, very like the rest of the penial appendix in the 

 Orthurethra.® Now, if this homology be correct, it is quite con- 

 ceivable that under certain circumstances Vallonia, and perhaps 

 also Acanthinula, might possibly secrete a dart in the penial appendix ; 

 and as the older authors thought that these snails were Helices, if 

 they did find a dart they might easily assume that the structure 

 in which they found it must be a dart-sac of the type that usually 

 occurs in the Helicidse. 



However this may be, it is clear that the evidence of the re- 

 productive system, taken as a whole, supports that of the other 

 organs which we have already considered. A classification that is 

 based on the study of a single organ, or even of a single group of 

 organs, is often unnatural, and should always be regarded with 

 suspicion ; but it is evident that those authors who have already 

 transferred Acanthinula from the Helicidse to the Orthurethra, on 

 account of the form of the genital ducts, have undoubtedly acted 

 rightly, and that not only Vallonia, but also Pyramidula and 

 Patulastra must certainly be placed in the Orthurethra as well. 



1 Ibid., pi. X, fig. 25, pi. xi, fig. 32. 



^ Journ. of Conch., vol. iv, p. 198, pi. viii, figs. 8, 9. 



* Op. cit., pp. 6, 8, 12, 13. 



* See Simroth, Journ. Coll. Sci. Tokyo, vol xii, 1898, p. 82. 



' See, for example, Wiegmann's figure of Pachnodus velutinus (Pfr.)in Mitth. 

 Zool. Samml. Mus. Berlin, vol. i, 1898. pi. iv, fig. 8. 



* A? in Staff or dia daflaensis Godwin-Austen, L. and F.W. Moll, of India, 

 vol. ii, 1907, pi. cxiii, figs. lA, li. 



