BOWELL : ON- HADUL^ OF VITREA SPP. 



59 



In order to study V. Rogersi, which is well knowa to be a very 

 variable form, it is desirable to have frequent access to a locality 

 where it abounds ; Mr. Kennard kindly indicated to me such a locality 

 near Westerham. There is in the Westerham specimens a considerable 

 amount of variation in the length of the mesocone of the central tooth ; 

 the central tooth is also frequently asymmetrical, and bears an additional 

 ectocone on one side. The exact shape of the admedians is also very 

 variable, as shown in the example figured (Fig. 1). "When, as in the 

 top row in that figure, there is a shortening of the teeth, a kind of 

 ridge is formed at the base of the tooth, clearly marking the process 

 as abnormal ; and this kind of abnormality has no tendency to be 

 repeated in series. One never finds the neat and distinct quadration 

 of the basal plate which is so strongly marked in helvetica. The basal 

 plates of centrals and admedians are much longer; the cusps of the 

 centrals are rounded or ovate ; mesocone of admedians more slender, 

 ectocones having a tendency to disappear, or to be reduced so as to 

 suggest a serration. It ought to be mentioned that with a microscope 

 of poor defining power the appearance of serration is often given where 

 there is merely a slight irregularity of contour, the eye not being able 

 to distinguish between true contours and interference effects ; the 

 pectinations on the teeth of Physa fontinalis, which are in reality thin, 

 equal, and parallel, though wedge-shaped in section, form a most 

 excellent test for the objective to be used for examining radulae. The 

 marginal teeth of F. Rogersi are comparatively short and not closely 

 set, totally unlike those of either glabra or helvetica. In general, the 

 variation of Rogersi is non- significant, being due mainly to looseness 

 of build. 



Fiq S 



196 



F-^-9 7 



Fig. 6 represents the proximal part of the genitalia of a Westerham 

 Rogersi, for comparison with Fig. 5 {helvetica). The duct of the 

 spermatheca, which is soft and flexible, containing less muscular fibre 

 than do the other canals, has been dissected out and is here shown laid 

 over the first half-turn of the female epididymis. The vas deferens 



