157 

 OBSERVATIONS ON THE RADUL2E OF 

 HYALINIA DRAPARNALDI, CELLARIA, ALLIARIA 

 AND GLABRA. 



By W. moss and A. E. BOYCOTT.^ 



(Read before the Society, January loth, 1906). 



Plate I. 

 The observations of which a summary is here given were made more 

 than five years ago. At the time it was hoped that it would be 

 possible to extend them to the other members of the group, and to 

 examine in more detail the growth and other variations. For various 

 reasons this has not been possible. That part of the enquiry which 

 reached the greater degree of completion may, however, be of sufficient 

 interest to other workers to be worth publication. It is certainly 

 desirable that it should be amplified and extended. 



In all more than 150 radulse were examined. In some of these a 

 fairly obvious mistake in specific diagnosis had been made, while 

 others showed pecuharities which rendered it undesirable to include 

 them in a series which was designed to be representative of specific 

 differences between closely allied species. For these and other 

 reasons the total number dealt with here is reduced to 131. 



The measurements of (3) length and (4) breadth are not to be 

 taken too minutely since the radula is often curved or folded 

 (especially anteriorly) and the degree of pressure used in mounting 

 no doubt makes a difference. This applies especially to the width 

 which (apparently) varies a good deal in different parts of the same 

 radula; in this series it has been measured at about the tenth row 

 from the posterior end. In reckoning the (6) number of rows, the 

 very youngest is not counted if it can only be seen (in glycerine-jelly) 

 with a very small diaphragm. In (7) number of marginals, the 

 maximum number found is given. As in the measurements of 

 individual teeth, the rows chiefly examined have been those imme- 

 diately in front of the young, pigmented part of the radula, i.e., rows 

 five to ten, and more especially rows six, seven and eight. The exact 

 number is not important; it varies in different parts and on the two 

 sides of the same radula and is dependent on the presence or absence 

 of the minute terminal denticles. To avoid confusion in description, 

 it has been reckoned throughout that there are three lateral teeth, 

 and the fourth from the central tooth is counted as the first marginal. 

 Owing to the difficulty of accurately defining the anterior margin of 

 the basal plate, it has been sought to give some idea of the compara- 

 tive size of individual teeth by measuring the distance from the tip of 



I With a few negligeable exceptions the whole of the material was prepared and mounted 

 by W.M., who is responsible for the determination of species ; the detailed 

 examination and measurements were undertaken by A.E.B. 



