PHYLLIVORA. 65 



" With reference to the actual size of the teeth, it 

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

 as the measuring standard ; and therefore the num- 

 bers 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 Arion ater the central and their neio-hbourino- 

 plates are 25 long by 15 wide. In Limux maximus 

 they are 20 long by 11^ wide. In Bulimus ob- 

 scurus the length of this plate is 7, while the average 

 length of all in the row is 4|. In Zua luhrica the 

 length is 5\, and the average breadth A\. In 

 Clausilia nigricans the length is 4^, and the average 

 breadth 3 f. The primary lateral plates of Limneus 

 stagnalis is 22 long by 12f wide. In Amphipeplea 

 glutinosa the corresponding plates are 11^ long and 

 10 wide, which happens to be precisely the size of 

 the primary teeth of plates in Planorbis corneus. 



" The teeth of Arionidcp and LimacidcB are much 

 alike, and differ from Helicidcs in having a long pro- 

 jecting single apex to the edge teeth. 



*•' The Helicidce, on the other hand, show a marked 

 disposition to increase the number of apices by bi- 

 furcation as they approach the edge. 



'^ Arion ater, Limax maximus, and L. carinatus, 

 Vitrina pellucida, Zonites alliarius, Z. cellarius, Z. 

 nitidulus, and Z. radiatulus have the edo^e teeth 

 aculeate ; Helix fulva, H. aspera,H. pomatia, Zonites 

 rotundatus, &c. have the edge teeth serrate." — 

 Thompson. 



The jaw in Limaces and Helices is single, in the 

 upper part of the buccal cavity, and is more analo- 



F 



