668 WHALES xxvi 



vertebrates, are rounded and mobile; they are the chief agents of 

 respiration, the diaphragm containing little muscle. 



In the fore-limb the humerus is short, the elbow-joint hardly 

 mobile, and the hand increased in length and sometimes expanded 

 (right whales, killers, river dolphins). The number of fingers is often 

 reduced to four; the phalanges of some of the digits may be con- 

 siderably increased in number (hyperphalangy). The scapula is 

 flattened and there is no clavicle. 



Some striking modifications are seen in the head. The skull shows a 

 curious telescoping of the bones over each other. The maxilla of 

 toothed cetaceans extends above the frontal, combining with the latter 

 to make a roof to the temporal fossa. Thus the maxilla almost reaches 

 the supra-occipital. In baleen whales the backward prolongation of 

 the maxilla is mainly below the supraorbital process of the frontal, 

 although there is a medial process extending dorsally towards the 

 supra-occipital. This telescoping occurs only towards the end of foetal 

 life. A further curious feature is that the skull in the toothed whales is 

 asymmetrical, the vertex being shifted over to the left side; no satis- 

 factory explanation for this phenomenon has yet been offered. The 

 jaws are always greatly elongated, in the Greenland right whales they 

 make up one-third of the total body length. The masticatory muscles 

 and the coronoid process are reduced, the latter most extremely in 

 Balaena in which it is distinguished only as an inconspicuous ridge. 



Whales are microsmatic or even in some species anosmatic (with no 

 olfactory nerve). The brain-case is therefore short and rounded while 

 the nostrils have moved backwards and open upwards. The nasal 

 bones have become reduced in length and no longer roof over the 

 nasal cavity. The process has proceeded somewhat differently in the 

 toothed and in the whalebone whales. The auditory region is much 

 modified and the whole petrosal bone is free from the rest of the skull ; 

 there is a large tympanic bulla, fused with the petrosal. Extensions of 

 the middle ear cavity form pneumatic sacs, below the base of the skull, 

 which serve to insulate sound and equilibrate the varying pressures 

 experienced under water. 



The feeding arrangements provide further special features. Many 

 of the toothed whales, such as the porpoises and dolphins, eat mainly 

 fish and the teeth form a row of numerous (65/58), similar peg-like 

 structures, usually in both jaws. With elongation of the jaws the 

 masticatory function of the teeth has been reduced and they probably 

 serve to hold the prey. Orcinus, the killer-whale, has large powerful 

 jaws and teeth and its diet includes dolphins, birds, seals, and the 



