WATSON : AFFINITIES OF PYRAMIDULA, ETC. 



15 



variation in the muscular system of the Ortliurethra. In all these 

 snails the retractor of the right upper tentacle passes between the 

 penis and the vagina. We see, then, that the evidence of the 

 cephalic retractors supports that of the radula, excretory system, etc. 

 The penial retractor of Vallonia and Patulastra arises from the 

 front end of the diaphragm, as in Ena obscura and Cochlicopa 

 lubrica. In Pyramidula and AcantJiinula,^ on the other hand, it 

 arises from the hinder end of the diaphragm, as it does in Lauria 

 cylindracea. 



5a. Pyr. ru.pe»tris 



6b, Pal. balwisi. 



5aL. G. Totuadatus 



5c. V pulcKella 



Figs. 5a-d. — Chief retractor muscles in Pyramidula, Patulastra, Vallonia, and 

 Goniodiscus. 



1. Retractor of left upper tentacle. 



2. Retractor of left lower tentacle. 



3. Retractor of lower part of left side of head and front end of foot. 



4. Retractor of buccal mass. 



5. Retractor of lower part of right side of head, genital atrium, and 



front end of foot. 



6. Retractor of right lower tentacle. 



7. Retractor of right upper tentacle. 



8. Retractor of hinder part of foot. 



9. Columellar muscle. 



Evidence op the Repeoductive System. — Admirable 

 descriptions and figures of the genital organs of the British species 



1 Steenberg states that in Acanthinula aculeata the penial retractor arises 

 from the columellar muscle {op. cit., p. 5) ; but Dr. Boycott's serial sections 

 show that this is not the case in British specimens. 



