IN TEETH. 49 



as in the instance of whales and fishes, the structural 

 features, by the aid of which the adaptation has been 

 brought about, are very different in the respective groups. 



In respect to the teeth and dentition, many very well- 

 marked instances of parallelism may be adduced. We 

 have already referred to the similarity of the tusks in dif- 

 ferent groups of mammals ; while in a previous chapter 

 we noticed the loss of upper front teeth, not only in the 

 modern ruminants, but likewise in the peculiar even-toed 

 ungulate known as Protoceras, of which the skull is figured 

 on page 53. The rhinoceroses likewise show a gradual 

 tendency to the loss of front teeth, resulting in the African 

 forms in the disappearance of the whole series from both 

 jaws. More remarkable, however, is the tendency to a 

 complete loss of the whole of the teeth in certain groups 

 of all the higher classes of vertebrates, as exemplified by 

 all modern birds, by turtles and tortoises, as well as certain 

 of the extinct flying dragons and fish-lizards among 

 reptiles, and by the great anteater, the scaly anteater, 

 and the echidnas, or spiny anteaters, among mammals ; 

 in all of which teeth are completely lacking. Moreover, 

 in many of these animals, such as birds, tortoises, and 

 probably the toothless flying dragons, the beak-like jaws 

 thus produced were sheathed in horn, thus showing a 

 kind of double parallelism, viz., the loss of teeth coupled 

 with the acquisition of a horny sheath to the jaws. 



Another instance of parallel development afforded by 

 teeth relates to the gradual heightening of the crown and 

 the production of a flat plane of wear not only among 

 several distinct groups of hoofed mammals, such as the 

 elephants, horses, and ruminants, but likewise among the 

 rodents. It is evident that these high-crowned teeth 

 have been independently evolved in the three great groups 

 of hoofed mammals, of which the above-named creatures 

 are typical representatives ; and it may be added that the 

 molars of horses and ruminants present a further evidence 

 of parallelism in their assumption of a more or less 

 decidedly crescent-like (selenodont) structure, the essential 

 peculiarities of which are described in the eighth chapter. 

 This resemblance between the molars of horses and 



