PHYLLIVORA. 63 



" In Limax carinatus, and some allied species, we 

 have this clearly exemplified, the changes from 

 the typical form into that at the edge being very 

 gradual, and showing every possible connecting link, 

 fig. 21. a d. But this gradual progression is far 

 from being the case in all species. For example, a 

 sudden and abrupt change in form is to be seen 

 in the fourth lateral tubercle of Zonites radiatulus, 

 fig. 22. c. This sudden change of form is owing to 

 the absence of the connecting links, which a refer- 

 ence to the progressive alteration in other species 

 will readily supply. Wherever a straight transverse 

 line is observable in the arrangement of the lateral 

 teeth, all the teeth in that line are similarly formed, 

 whether the right or left laterals are in the same 

 line, as in Planorbis contortus, fig. 18. ., or divaricate 

 from each other at the central tooth upwards, as in 

 Achatina acicula^ fig. 18. d. 9 or downwards, as v&Ancy- 

 lus fluviatiliS) fig. 18. b. Wherever the curve presents 

 great angularity, as in Zonites radiatulus, fig. 18. c., 

 there we find a sudden change in the form of the 

 teeth, while in like manner a gradual curve (Limax 

 carinatus, fig. 18. gJ) 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. 



" The following table gives some idea of the 

 number of teeth in a transverse or horizontal row, 

 of a few species, together with the number of rows 

 upon the tongue, and the whole number of teeth on 

 that organ. 



