The Dentition of British Mollusca. 71 



show a great similarity in their dentition. Their tongues are 

 <e like a tesselated pavement/' so regular are their numerous 

 teeth. These are mostly rectangular in ground-plan, and 

 armed with a single (or sometimes triple) recurved point. 

 They are often so very minute, that their characters are barely 

 discernible, even by the aid of the best lenses. When this 

 happens, we may avail ourselves of the rule established by 

 Mr. W. Thomson, who first in England directed attention to 

 this subject. He found that the form of the whole transverse 

 row corresponds to certain peculiarities in the teeth, to such an 

 extent as to be an almost equally safe guide in questions of 

 affinity. Thus, each row passes straight across the tongue in 

 Planorbis albus and vortex, is curved in Limax marginatus, 

 and suddenly bent in Zonites cellarius. Whence it may be 

 inferred that the teeth are all similar in cases like the first 

 named, and gradually or suddenly differ in the others res- 

 pectively. 



It is among the in-operculated members of the order Pul- 

 monifera that we meet with the most astonishing instances of 

 large numbers of teeth. Limax mazimus possesses 27,000, 

 distributed through 180 rows of 160 each. Helix pomatia has 

 21,000; and its comparatively dwarfed congener, H. obvoluta, 

 no less than 15,000. When it is remembered that these 

 estimates refer to series of forms, often elegantly curved and 

 sculptured, the total area sustaining them not measuring at the 

 utmost more than half an inch long and one- eighth broad, we 

 must be filled with admiration at the marvellous prodigality of 

 the great creative power thus bestowed upon such a small part 

 of the organization of an humble snail. And when I ask my 

 readers to examine these things for themselves under the 

 microscope, I venture to think that the varied and beautiful 

 outlines and serried ranks of these delicate amber- coloured 

 atomies will be viewed with a delight whose depth and intensity 

 the observers of nature can alone rightly measure. 



The examples figured in the plate are drawn from 

 original preparations,* and represent the principal types. 

 And in the following table I have placed the genera 

 known to me under their respective formulae, as some 

 guide to the student of these objects. The group charac- 

 terized by the numbers 1.1.1 will be noticed as the best, the 

 animals being all flesh-eaters, with the exception of, per- 

 haps, Lamellaria. The generic names are those employed by 

 Forbes and Hanley, in their British Mollusca. 



* Those of Lamellaria, Doris, Goniodoris, and Eolis papillosa are from pre- 

 parations kindly lent me by Mr. lirady, York. „, 



