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



that the number of teeth upon a tongue is never likely to be of 

 more than specific value as a characteristic feature, since there 

 appears to be no general number, or even approximate number, 

 which can be said to belong to any genus. Since Limax maxi- 

 mus heads the list with 27,000, and Helix Pomatia follows with 

 21,000, it might be conjectured, perhaps, that size had some in- 

 fluence in the matter ; but then we find Helix aspersa and nemo- 

 ralis possessing nearly the same number, while Helix obvoluta, 

 a shell very little, if at all, larger than Zonites cellarius, possesses 

 more than fifteen times the number of teeth. 



With reference to the actual size of some of the teeth, it will 

 be most convenient to take the 10,000th of an inch as the 

 measuring standard; and therefore the numbers that I shall 

 now use, in giving the dimensions of the teeth, are to be regarded 

 as so many 10,000ths of an inch. 



In Avion ater, the central and neighbouring plates are 25 long 

 by 15 wide. In Limax maximus they are 20 long and 11^ wide. 

 In Bulimus obscurus the length of the plates is 7, while the 

 average breadth of all in the row is 4|. In Zua lubrica the 

 length is 5i and the average breadth 4^. In Clausilia nigricans 

 the length is 4^ and the average breadth 3|. The primary 

 lateral plates of Limnaus stagnalis are 22 long by 14^ wide. In 

 Amphipeplea glutinosa, the corresponding plates are 11 A long 

 and 10 mde, which happens to be precisely the size of the 

 primary lateral plates in Planorbis corneus. 



King's College, July 1850. 



EXPLANATION OF PLATE IV. 



Fig. 1. Central portion of transverse row o{ Zonites radiatulus: a, central 

 tooth ; b, first lateral ; c, fourth lateral tooth. 



— 2. Head of a Snail : a, horny tooth ; b, oesophagus ; c, tongue. 



— 3. Diagram of a vertical section of a tooth. 



— 4. Ditto central tooth. 



— 5. Limax carinatus : a, central tooth ; b, first lateral. 



— 6. Ditto an intermediate lateral tooth. 



— 7- Ditto edge-tooth. 



— 8. Zua lubrica (a, b, as above). 



— 9. Planorbis carinatus (a, b, as above). 



— 10. Ditto an intermechate lateral tooth. 



— 11. Ditto edge-tooth. 



— 12-19. Direction of transverse rows : — 



— 12. Planorbis contortus. Fig. 16. Zua lubrica. 



— 13. Ancylus fluviatilis. — 17- Vitrina pellucida. 



— 14. Zonites radiatulus. — 18. Limax carinatus. 



— 15. Achatina acicula. — 19. Helix obvoluta. 



