90 Mr. W. Thomson on the Dentition of British Pulmonifera. 



curve presents great angularity (as in Zonites radiatulus) (fig. 14), 

 there we find a sudden change in the form of the teeth, while 

 in hke manner a gradual curve is the result of a gradually pro- 

 gressive change in the form of the teeth, the degree of deviation 

 from a straight line being exactly in proportion to the amount 

 of change which takes place between the form of the central and 

 edge-teeth. 



It may perhaps seem that I have dwelt at greater length on 

 this point than was necessary ; but as there are many species of 

 Pulmonobranchiata so small as to render it difficult with the best 

 glasses to determine the form of the plate and often of the 

 tubercle, the attachment of the tubercle to the plate being the 

 only part clearly visible, it appears to me desirable that the fol- 

 lowing rules should be laid down with reference to the form of 

 the lateral teeth, in connection with the horizontal rows. 



A straight line indicates similarity in the teeth ; a curve in- 

 dicates a GRADUAL change in their form, and an angularity in 

 the row indicates a sudden change. 



Having stated in general terms what are the usual charac- 

 teristics of the teeth amongst the Pulmonobranchiata, I purpose 

 now to offer a few observations upon those variations in them 

 which seem to be characteristic of certain genera and families ; 

 prefacing these remarks with a list of the species I have had an 

 opportunity of examining. 



Arion ater. Bulimus obscurus. 



Limax maximus. acutus. 



carinatus. Zua lubrica. 



Vitrina pellucida. Achatina acicula". 



Helix aspersa. Pupa marginata. 



hortensis. jiiniperi. 



nemoralis. Vei-tigo edentiila. 



Pomatia. pygmaea. 



arbustorum. Balsea pen'ersa. 



obvoluta. Clausilia bideus. 



lapicida. nigricans. 



pulchella. Carjchium minimum. 



Cantiana. Limnaeus pereger. 



Cai-thusiana. stagnalis. 



fulva. palustris. 



concinna. Amphipeplea glutinosa. 



. Pisana. Ancylus lluviatilis. 



virgata. Velletia lacustris. 



caperata. Physa fontinalis. 



ericetomm. Planorbis corneus. 



Zonites rotundatus. albus. 



alliarius. carinatus. 



cellarius. marginatus. 



nitiduUis. nitidus. 



radiatulus. contortus. 



Succinea putris. Segmentina lineata. 



Bulimus Lackamcusis. Cyclostoma elegans. 



