Mr. W. Thomson on the Dentition of British Pulmonifera. 91 



Since this list comprises httle more than half our British 

 species, it would be hazardous to attempt to deduce any positive 

 theories as to the constancy of form in any particular groups. 

 Indeed, it would not be safe to lay down any rules even from 

 an examination of all the British species, since many in the 

 same genus present such marked differences, alike in external 

 form and in the conformation of their teeth, that it would be 

 impossible to arrive at any satisfactory deductions, without the 

 opportunity of examining the connecting links which foreign 

 species will supply. 



In the list I have given, the names and arrangement are 

 those used in the last edition of Turton's ' Manual,' and on the 

 whole, the form of the teeth is confirmatory of this classification. 

 The Arionidce and Limacidce are much alike, and differ from the 

 Helicidce in having a long projecting single apex to the edge- 

 teeth. The Helicidce, on the other hand, show a marked dispo- 

 sition to increase the number of apices by bifurcation as they 

 approach the edge. 



Arion ater. 

 Limax maximus. 

 carinatus. 



Yitrina pellucida. 

 Zonites alliarius. 



cellarius. 



■ nitidulus. 



■ radiatulus. 



Helix fulva. 



> Edge-teeth aculeate. 



aspersa. 



Pomatia. )> Edge-teeth serrate. 



Zonites rotundatus. 

 &c, &c. 



Vitrina evidently belongs more to the Limacidee than the 

 Helicidce, as is shown by the single prolonged apex to the edge- 

 teeth. From the very similar character of the edge-teeth in 

 Zonites alliarius, ccllarms, nitidulus and radiatulus (whose tongues 

 greatly resemble each other), I am induced to believe that they 

 should come in between Vitrina and the true Helices, for while 

 their edge-teeth show no appearance of bifurcation, the heel to 

 the apex may possibly be looked upon as an approach towards 

 it. Their sagittate central tubercle corresponds with that of 

 Vitrina, and a similarly-shaped central tubercle in Helix fulva 

 connects them with the true Helices, which have a simple aculeate 

 tubercle. Zonites radiatus (or rotundatus) is a true Helix. 



Succinea putris, from its partiality for the leaves of plants 

 growing in the water and for other very wet places, might 

 possibly be expected to show some change towards the form 

 of a Limnceus in its teeth, whereas on the contrary they are 



