1907. Boweix. — On \< 7 itrca (Hyalinia) hibemica. 331 



The radulse of hibemica and cellaria I have drawn in fig. 3. 

 The compact oblong centrals are found consistently in 

 V. hibemica : in V. cellaria one occasionally finds an approxi- 

 mation to this form, but chiefly in specimens exhibiting 

 strong marks of irregularity. The greater thickness and 

 prominence of the teeth on the outer rows is a character well 

 defined in all my specimens. 



The accompanying figures are all tracings made with the 

 Abbe drawing apparatus. The radulae were traced with the 

 Zeiss 3 mm. apochromat, compensating ocular 4. It may be 

 worth while to mention that by the use of these lenses an 

 absolutely flat field is obtained, which is indispensable for the 

 present purpose. A curvature so slight that it is inappreciable 

 for ordinary visual purposes is enough to make the accurate 

 drawing of radulae of this type an impossibility. Figures 3A 

 and b are to be regarded merely as transcripts of fact : they 

 have not been conventionalised in any way. In the lowest 

 row of figure 3B a "tooth" is shown which does not lie parallel 

 to the axis : it is consequently slightly foreshortened. I have 

 drawn these figures with the points of the teeth upward, and 

 I wish them to be so examined. The convention to the 

 opposite effect is due, I suppose, to the idea that the normal 

 position of the snail in the dissecting dish is the position of 

 running away from the operator. Be that as it may, I am 

 convinced that I am not alone in the feeling that the best 

 way of looking at a conical object is to look at it with the 

 points turned up and not down. A drawing of St. Paul's 

 Cathedral, for example, may be equally good upside down, 

 but it appears to me that it is not equally intelligible. 



The general impression left with me by the study of these 

 Vitreae is that V. hibet?iica plays dominant to V- cellaria's 

 recessive. V. hibemica, however, appears to be a pure 

 dominant, and probably also fixed : while V. cellaria includes 

 not only pure recessives, but — partly owing to its debased 

 gametic habits — a large number of mixed forms, among which 

 examples resembling the pure dominants, may occasionally 

 crop up. I hasten to add that I alone am responsible for this 

 speculation ; but it seems to me to justify Mr. Kennard's name 

 Vitrea h ibem ica , 



