766 MAMMALIA. 



in size from that of a sheep to that of a rhinoceros, form the sub-order 

 Toxodontia. 



Cope includes a number of generalised Eocene Ungulates under 

 the title Condylarthra. Some seem ancestral to the Perissodactyla 

 and Artiodactyla ; some suggest a union of ancestral Ungulates and 

 ancestral Carnivores. The genus Feriptychus may be regarded as 

 an ancestral Bunodont, and Phenacodus as near the origin of the 

 horse stock. But Phenacodus is so generalised that Cope suggested 

 affinities between it and not only Ungulates, but also Carnivores and 

 Lemurs. 



From the Eocene of N. America, Marsh disentombed a group 

 of animals which he called Tillodontia, e.g. Tillotherium, which 

 seem to combine the characters of the Ungulata, Rodentia, and 

 Carnivora. 



Few orders of Mammals are of more interest to the palaeon- 

 tologist than the Ungulates. Not only are fossil representatives 

 numerous, but their usually large size, and the fact that the teeth 

 are frequently an index of general structure, makes the determina- 

 tion of affinities much easier than in most cases. In consequence, 

 problems like that of the origin of the horse, or the relations of the 

 different proboscidians, have been worked out with a completeness 

 rare elsewhere. 



Order CETACEA 



The Cetaceans, including whales and dolphins and 

 their numerous relatives, are aquatic mammals of fish-like 

 form. 



The torpedo-shaped body has no distinct neck between 

 the relatively large head and the trunk, and tapers to a 

 notched tail, horizontally flattened into flukes. The fore- 

 limbs are paddle-like flippers, and there are no external 

 hints of hind-limbs beyond mere button-like knobs in some 

 embryos. Most forms have a median dorsal fin. Hairs 

 are generally absent, though a few bristles may persist near 

 the mouth. The thick layer of fat or blubber beneath 

 the skin retains the warmth of the body, and compensates 

 for the absence of hair. In one of the dolphins dermal 

 ossicles occur, a fact which has suggested the idea that the 

 toothed whales may have had mailed ancestors. Traces 

 of dermal armour have also been found in the extinct 

 Zeuglodonts. 



The torpedo shape, the absence of external ears, the 

 absence of an eye-cleansing nictitating membrane, the dorsal 

 position and valvular aperture of the single or double nostril, 

 the sponginess of the bones, the retia mirabilia storing 



