CETACEA. 



709 



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 mem- 

 brane, the dorsal position and 

 valvular aperture of the single 

 or double nostril, the sponginess 

 of the bones, the retia mirabilia 

 storing arterial blood in different 

 parts of the body, may be asso- 

 ciated with the aquatic life. 



The cervical vertebrae are thin, 

 and more or less fused. There 

 is no union of vertebras to 

 form a sacrum, for the hind- 

 limbs are at most very rudiment- 

 ary. Under the caudal vertebrae there are wedge-shaped 

 chevron bones. 



The brain-case is almost spherical ; the supraoccipital 

 meets the frontals and shuts out the parietals from the roof 

 of the skull ; the frontals arch over the orbit ; the snout or 

 rostrum of the skull is composed of premaxillas, maxilla?, 

 and vomer, and of the mesethmoid cartillage. The periotic 

 in whales is an exceedingly dense bone, and is of interest 

 because it is the only part of the skeleton found at great 



Fig. 313. — Left fore-limb of 

 Bal&noptera. 



Sc, Scapula with spine (sft.) ; H., 

 humerus ; R., radius ; U., ulna ; 

 C, carpals embedded in matrix ; 

 Mc, metacarpals ; Ph., phal- 

 anges. 



