BOWKLL : KADUL^ OF BRITISH HKLICIDS. 159 



mesocones and basal plates, for at this margia no line of demarcation 

 can be drawn between them. In consequence the external pleurae 

 present the appearance of numerous little crescents, sloping away 

 from theadmedian regions, and there is a good deal of overlapping, on 

 account of the unusual convexity of the radular membrane. There is 

 apparently a tendency to true longitudinal fusion of these externals. 

 The externals are highly pectinate, the pectinations being more 

 frequent and more filiform in lamellata. 



This genus also shows great variation in the form and proportion of 

 the central and admedian cones. Lamellata can be known at once hj 

 the presence of an additional small cone between the admedian 

 mesocone and its ectocone. (Similar structures are found in Punctum ; 

 they are extremely prevalent in the genus Sicccinea.) The mesocones 

 of the more lateral adraedians of both species are long and lancet- 

 shaped, and have a tendency to slant inwards. Aculeata has a very 

 narrow central, that of lamellata being distinctly wide. They closely 

 resemble those of Pimdum jrygmaum and Vertigo moulinsiana 

 respectively. As in the Vallonice and in Piinctnm, the actual number 

 of unci on" each side of the central is small— from thirteen to sixteen 

 in adult specimens. 

 It 



1^. 



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With regard to rotimdata, rnpestris, and pygmcBum, it may be 

 repeated that each has a most characteristic ladula, and each exhibits 

 a distinct type. If we examine the basal plates we find that in 

 Punctum ptgm^um the centrals are like those of a Succinea, and this 

 resemblance is continued as we trace the rows outwards on the y)leurge. 

 All the basal plates are sub-triangular, the inner side being slightly 

 convex and having a tendency to be slightly notched ; and the size 

 of the basal plate of the first admedian and that of the last (sixteenth) 

 uncus is approximately the same. A secondary pleural appearance, 

 probably not indicating a true pleural division, is found after the 

 ninth row from the centre. The ninth admedian is the last which 

 has its basal plate pointing straight backwards to the uncus of the 

 succeeding row ; that of the tenth admedian points to the space 

 between the tenth and eleventh uncus of the row behind it ; the 

 eleventh points straight at the twelfth succeeding uncus, and this 

 new parallelism is maintained to the end, so that the sides of the 

 radula acquire a fan-like appearance. Simultaneously the angle of 

 insertion of the cones is varied so as to increase tlie illusion. This 

 arrangement is quite typical in Succinea. It is also found in 

 Ovthaliciis. In the Vertiginidse we find some radulse quite close 

 to iYmt oi Punctum, closely allied species having the organ developed 



