Chap. IV.] 



TEETH OF VERTEBRATES. 



141 



jaw of a dogfish, at a stage previous to that at which any 

 lip is developed, shows the direct continuity of struc- 

 tures, which, in the adult, seem to be very different 

 from each other. When we consider the different re- 

 lations to the surrounding parts which would be entered 

 into by the spines that 

 came to lie within 

 the area of the mouth, 

 we see at once that, 

 by being brought into 

 contact with the food 

 the spines would be 

 led to increase in size 

 and strength ; but this 

 increase in activity 

 would be necessarily 

 accompanied by a 

 richer nervous and 

 vascular supply; and 

 this, reacting on the 

 spines, would lead to 

 greater differentia- 

 tion, which has taken the form of greater definiteness 

 in arrangement and structure. 



In commencing, therefore, a review of the teeth 

 of vertebrates, we find that we start with a general- 

 ised or non-differentiated condition ; as we pass on 

 we shall find that the teeth become more and more 

 limited to certain bones, and diminish in number ; in 

 other words, there is a gradual reduction. Concur- 

 rently with this, we have to note that, when a group 

 of animals becomes especially adapted to a certain 

 mode of life, or presents marked aberrations from the 

 general plan of structure, they become edentulous, 

 or lose their teeth ; such, for example, are the pipe- 

 fishes among Fishes, toads among Amphibia, Chelonia 

 (turtles and tortoises) among Reptiles, all recent 





of 



young Dog-fish, showing the spines 

 of the skin under the jaw, and the 

 teeth above. (After C. S. Tomes.) 



