l62 



NATURE 



[Dec. 21, 1876 



that a thin layer of this material is to be found in snakes, 

 and that an enamel organ is always present, as in the 

 frog and armadillo, where in the fully-formed tooth this 

 tissue cannot be detected. We cannot refrain alsa from 

 quoting a sentence with reference to a point of zoological 

 interest which is frequently overlooked, for, says the 

 author — " as was pointed out by my father, the passage 

 of the dentinal tubes into and through a great part of 



Fig 4. 



the thickness of the enamel, takes place in marsupials 

 with such constancy, as to be almost a class character- 

 istic." This condition is seen in Fig. i, where A is 

 the dentine, B the enamel, and C one of the dentinal 

 tubes which enters the latter at the point where its course 

 is most bent, and the smaller lateral ramifications cease. 



It is with regard to the development of the teeth that 

 Mr. Tomes gives us most valuable information, especially 



Fig. 5. 



among the fish, amphibia, and reptiles, and his argument 

 as to their homologies, deduced from their condition in the 

 sharks, is so well stated that we cannot refrain from 

 quoting it. We read that " if a young dog-fish just about 

 to be hatched be examined, it will be found that it has no 

 distinct under lip, but that its skin turns in under its 

 rounded jaw without interruption. The skin outside 



carries spines (placoid scales),, and these spines are con- 

 tinued over that part of it which enters the mouth and 

 bends over the jaws ; only they are a little larger in this 

 latter position. If the growth of the dog-fish be followed, 

 these spines of the skin which cover the jaws become 

 developed to a far greater size than those outside, and the 

 identity and continuity of the two become to some extent 

 masked. No one can doubt, whether from the com- 

 parison of the adult forms or from a study of the deveftop- 

 ment of the parts, that the testh of the shark [and dog- 

 fish] correspond to the teeth of other fish, and these 

 again to those of reptiles and mammals ; it may be clearly 

 demonstrated that the teeth of the shark are nothing 

 more than highly developed spines of the skin, and there- 

 fore we infer that all teeth bear a similar relation to the 

 skin. This is what is meant when teeth are called dermal 

 appendages, and are said to be perfectly distinct from the 

 internal bony skeleton." 



We reproduce a woodcut (Fig. 2) representing a trans- 

 verse section of the active and reserve poison-fangs of a 

 viper, in which i is the tooth in use, the other numbers 

 tieing affixed to tho-se which will succeed when that 

 has dropped or been withdrawn by mechanical violence 

 in the order of their succession, they being arranged, as 

 can be seen, in pairs. It is shown that this system of 



Fig. 6. 



paired series does not exist in the cobra, in which the 

 successional teeth form but a single series ; and it is 

 suggested that "perhaps this may serve to explain the 

 preference of the snake-charmers for the cobra, which 

 would probably take longer to replace a removed poison- 

 fang th an a viperine snake would." 



The striking resemblance between a transverse section 

 of the dentine in the Ray Myliobates and the Edentated 

 maifamal, the Aard-Vark IjDrycteropus) of South Africa is 

 well illustrated in Fig. 3 ; as in Figs. 4, 5, and 6 are the 

 steps, through the Monitor Lizard (Fig. 4), and the 

 American Bony Pike (Fig. 5), by which a tooth, apparently 

 so elaborate as that of the extinct Labyrinthodon (Fig. 6) 

 — consequently so named by Prof. Owen — may be assumed 

 to have been arrived at by a gradual increase in the 

 number and the depth of the numerous inflections of the 

 pulp-papilla. 



Mr. Tomes takes an extremely broad view of the mam- 

 malian tooth series, and,^ instead of following the but too 

 frequent method of specialists, is willing to admit that 

 similarity in dentition is not always associated with classi- 

 ficational afifinity. As an instance of this we read that 

 *' it is very easy for us to see how a rodent type of denti- 

 tion is beneficial to its possessor by rendering accessible 

 articles of food wholly unavailable for creatures which 

 have no means of gnawing through a shell or other hard 

 body. Now it happens that in three regions of the world, 



