WATSON : AFFINITIES OF PYRAMIDULA, ETC. 17 



not leave any rooni for accessory organs that are not absolutely 

 necessary. 



Pahdastra balmei is also viviparous, and in this species there 

 seems to be no trace of a penial appendix, as is the case in the British 

 species of Azeca and in many of the Pupillidse. But the absence of 

 an appendix is fully counterbalanced by the remarkable complexity 

 of the internal structure of the epiphallus and penis (pi. I, fig. 3). 

 A well-marked epiphallus is also developed in the other three 

 genera that we are considering, and in Acanthinula aculeata it bears 

 a couple of extremely short, thick flagella. These are very different 

 from the slender flagellum of Helix — very unlike " little whips " — 

 but similar flagella occur in some of the Enidae and Pupillidse. 



We see, therefore, that Pyramidula, Patulastra, Acanthinula, 

 and Vallonia agree closely with the Pupillidse, Enidse, and their 

 allies in their male genital ducts — when these are present. Boycott 

 and Steenberg, however, have shown that in all the specimens 

 of Acanthinula lamellata that they examined, the penis, epiphallus, 

 etc., were entirely absent, and Dr. Boycott found that the same 

 was true of about half of the full-grown examples of A. aculeata 

 that he studied. In both species the first part of the slender vas 

 deferens is present beside the oviduct, but in these individuals 

 it stops abruptly at about the level of the anterior end of the 

 receptacular duct, and not a trace of the rest of the male organs 

 exists. The physiological significance of this remarkable phenomenon 

 has been so ably discussed by Dr. Boycott that I need not deal 

 with it again. From a purely systematic point of view it is of more 

 interest to point out that the same phenomenon occurs in Vallonia. 

 I have made a very careful examination of the genital ducts of no 

 fewer than 98 full-grown specimens of Vallonia, 45 being examples 

 of V. costata, 31 of V. j)ulchella, and 22 of V. excentrica. All the 

 examples of V. pulchella, and most of those of the other two species 

 were collected in Cambridgeshire ; about half were examined in the 

 spring, but 26 specimens of V. costata, 12 of V. pulchella, and 10 of 

 V. excentrica not until November. Of all these specimens only three 

 examples of V. costata had any male organs, two being found in 

 November and the other one in the spring. In the remaining 

 95 individuals the female ducts were well developed, but there was 

 no trace of the male ducts ; even the first part of the vas deferens 

 could not be found, but the reproductive organs of all three species 

 closely resembled pi. II, fig. 6. The fact that this unusual 

 phenomenon occurs in both Vallonia and Acanthinula supports the 

 view that these two genera are closely related to each other and to 

 the Pupillidse, for the same phenomenon occurs in at least one 

 member of that family, namely. Vertigo moulinsiana} 



^ It has been suggested that Vertigo should be placed in a separate family, 

 since it has no lower tentacles (Kennard & Woodward, List of British Non- 

 Marine Mollusca, 1914, p. 2) ; but in most respects the anatomy of this genus 



VOL. XIV. — APKIL, 1920. 2 



