156 



ON THE RADUL.E OF THE BRITISH HELICIDS. PART IV. 

 By llev. E. W. BowELL, M.A. 



Read Sth May, 19U. 



Three parts of this paper appeared iu vol. viii of these Proceedings. 

 Part III, together with the present contribution, will be found to 

 contain tigures of all the species referred by Dr. Gwyu Jeffreys to the 

 genus Helix} All the larger species, except Theba cartnsiana and 

 Ifi/f/romia fusca, liave already been figured and described. 



Theba cartusl^na. — The central and admedian unci differ in the 

 following minor points from those of Th. cantiana : — 



1. The central uncus is proportionally larger. 



2. The ectocones are less prominent. 



3. The apices are shorter and have less sinuosity of outline. In 

 the more lateral admedians the apices tend to become pointed. Tlie 

 apical fossa is not conspicuous. 



4. The posterior margins are decidedlj' less prominent. 



5. There is a well-marked indentation of the lacinia. 



The external unci are remarkably different in appearance. This is, 

 however, mainly due to their cones being produced into regular 

 styloid processes. If the average proportion of width to length be 

 represented as about 2 : 3 in cantiana, in cartnsiana it reaches 1 : 4. 

 Also, the apices are markedly triangular. The median part of each 

 uncus overlaps tlie lateral part of the next, but there is no fusion. 



The diagnosis of cartusiana is therefore easily made, but it is clearly 

 of the same type as cantiana. The great difference between the 

 proportions of the external cones furnishes another instance of the 

 principle that prominence in any given character takes the forms of 

 excess and defect in allied species. On comparing these two forms 

 I am disposed to think cartusiana to be the earlier, because there is 

 a distinct tendency to thickening of the cone elements, especially on 

 the externals, in the more developed forms. The embryonic cantiana 

 exhibits externals more nearly resembling those of cartnsiana. 



It may be suggested that habitat and food have played their part 

 in determining the form of these unci. I have only once found 

 cartusiana : the animals were feeding upon thistles in an exposed 



^ The radula of Vallonia pulchella (Miill.) has not been figured, as I was 

 unable to obtain specimens from shells which belonged without doubt to 

 this species. 



