14 PROCEEDINGS OF THE MALACOLOGICAL SOCIETY. 



to become pectinate, the number of cusps being increased to com- 

 pensate for the reduction in the number of separate teeth. But 

 in the Sigmurethra the innermost cusp of these pectinate marginal 

 teeth is formed by the endocone instead of the mesocone. This is the 

 case, for example, in Helicodiscus lineatus (see text-fig. 4e), and in 

 Clausilia hiplicata, which has, perhaps, the most distinctly pectinate 

 marginal teeth of our native Sigmurethra. In the Orthurethra, 

 on the other hand, not only are pectinate teeth extremely common, 

 but they are always of the type found in Pyramidula, Patulastra, 

 Acanthinula, and Vallonia, that is to say, they are pectinate teeth 

 without endocones. So far as I am aware, distinct endocones never 

 occur in orthurethrous snails. 



The marginal and lateral teeth of the four genera that we are 

 considering are exceedingly like those occurring in many of the 

 genera of the Pupillidae, and they als6 greatly resemble those found 

 in the Cochlicopidae and Amastridse ; moreover, they only differ 

 very slightly from those occurring in the less specialized members 

 of the Enidse. Pyramidula resembles the Pupillidse in its central 

 tooth being large ; Patulastra, Vallonia, and Acanthinula 

 aculeata agree with Cochlicopa, Azeca, and Leptachatina, in having 

 small, narrow central teeth ; while the intermediate size in the 

 central of Acanthinula lamellata is what we also sometimes find 

 in the Enidse. In short, the type of radula found in Pyramidula, 

 Patulastra, Vallonia, and Acanthinula differs from that found in 

 any of the sigmurethrous families, but agrees very closely with that 

 which characterizes the less specialized genera of the Orthurethra. 



The remainder of the alimentary canal is of the ordinary type, and 

 does not appear to present any features of much systematic 

 importance. It may be worth mentioning, however, that the species 

 of Vallonia and Acanthinula resemble Cochlicopa luhrica and Ena 

 ohscura in having the salivary glands united with each other below 

 the oesophagus, and not above it — a rather unusual arrangement — 

 and also that Pyramidula rupestris differs from Patulastra halmei 

 and the three species of Vallonia in that the most posterior of the 

 three lobes of the anterior division of the liver is without the dorsal 

 extension which usually runs forward beside the suture, between 

 the last part of the intestine and the albumen gland, in front of the 

 stomach (compare pi. I, fig. 3, with Steenberg, Vidensk. Meddel. 

 fra Dansk naturhist. Foren., vol. Ixix, 1917, p. 12, fig. 7,/"). 



Evidence of the Retractor Muscles. — It will be seen from text- 

 figs. ba~d that the branching of the columellar muscle is very similar 

 in Vallonia, Patulastra, and Pyramidula ; but that it is quite different 

 in Goniodiscus rotundatus, particularly as regards the origin of the 

 buccal retractor and the retractors of the lower tentacles. In such 

 forms as Lauria cylindracea, Ena ohscura, and Cochlicopa luhrica, 

 however, the arrangement of these muscles is practically identical 

 with that found in Vallonia, there being, apparently, very little 



