308 TOWARDS THE HUMAN FORM 



appearance of a horse walking on its fetlock- joints, and were, 

 indeed, almost plantigrade. They had only three toes on their 

 feet, terminating in enormous nails. How, we may now ask, 

 did they lose their lateral toes ? Were they descended from 

 climbing or burrowing animals ? We do not know. 



All Edentata are characterized by the remarkable develop- 

 ment of the skeletal system, which presents a curious contrast 

 to the abortion of the teeth. The same contrast is manifested 

 in Ornithorhyncus and the Echidna. Can we, therefore, 

 suppose that this enrichment of the skeleton by the lime salts 

 was accomplished at the expense of the dental system, thus 

 rendered comparatively inert ? 



This massivity of the skeleton coincides in the Sirenians 

 with an analogous reduction of the teeth. The Eocene 

 Prorastomus, indeed, had almost too many, since in addition 

 to the normal number of incisors and canines it had eight 

 molars instead of seven in each half -jaw. Halitherium, 

 likewise Eocene, which had preserved the rudimentary femur 

 of its hind limb, had already lost two incisors and the supple- 

 mentary molar. Only the male Dugong has functional 

 incisors, and four of its six molars are rudimentary, while in 

 the Sea-Cow the two incisors remain concealed under a horny 

 plate, but the number of molars of each half -jaw increases, as 

 in the armadillos, and reaches eleven altogether, of which six 

 only are functional. Finally, in the adult Rhytina, the teeth 

 have been replaced by horny plates, as in Ornithorhyncus, 

 These great animals had already been exterminated by 1768, 

 twenty-five years after their discovery. 



The dentition of the Cetacea has undergone similar 

 vicissitudes. No fossil forms are known that present a 

 dentition like that of the primitive placental Mammals. 

 Zeuglodon seems to approximate rather to the Seals. From 

 the very beginning, as the jaws elongated, the molars seem 

 to have become dissociated and to have returned to the 

 conical form seen in Reptiles. Only the Miocene Squalodon 

 shows a differentiation of the teeth into incisors, canines, and 

 molars. At that time, however, there already existed Dolphins 

 whose teeth were all of the same pattern ; Cachalots which 

 had none except in the lower jaw, and species of Hyperoodon 

 which had only one pair of teeth at the free end of the mandible ; 

 as well as a large number of Bakenoptera, or even of Whales 



