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 /erdptychus 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. : F 
From the Eocene of N. America, Marsh disentombed a group 
of animals which he called Tillodontia, eg. Z2l/otherium, which 
seem to combine the characters of the Ungulata, Rodentia, and 
Carnivora. 
Few orders of Mammals are of more interest to the palzeon- 
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 spindle-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 spindle 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 
