CRT ACE A. 



705 



Bunodont, and Phenacodus as near the origin of the horse stock. But 

 Phenacodus is so generalised that Cope has suggested affinities between 

 it and not only Ungulates, but also Carnivores and Lemurs. 



The tertiary strata of S. America have yielded a number of strange 

 types, e.g.) Toxodon^ Nesodon, and Typotherium, ranked in the sub- 

 orders Toxodontia and Typotheria. 



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

 animals which he calls Tillodontia, e.g., Tillotherium, which seem to 

 combine the characters of the Ungulata, Rodentia, and Carnivora. 



Order 4. CETACEA. 



The Cetaceans, including whales and dolphins and their 

 numerous relatives, are aquatic mammals of fisriTlike form. 



The spindle-shaped body has 

 no distinct neck between the 

 relatively large head and the 

 trunk, and tapers to a notched 

 tail, the horizontal expansions of 

 which form the flukes. The fore 

 limbs are paddle-like, and there 

 are no external hints of hind 

 limbs. 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 serves 

 to retain the warmth of the body, 

 and thus 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 general 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 mirabilta in different parts of the body, 

 may be associated with the aquatic life of these mammals. 

 45 



FIG. 258. Left fore-limb 

 of Balsenoptera. 



Sc. , Scapula with spine (sp) ; 

 //"., humerus ; fi., radius; /., 

 ulna; C., carpals embedded in 

 matrix; Me., metacarpals ; Pk., 

 phalanges. 



