THE NECK 



suits that were utterly discouraging. It may be put down partly to the 

 variation in the cervical needs of different sorts, and another possible ex- 

 planation is furnished by the fact that a very few species (as Neomeris) 

 develop a slender, short pair of accessory ribs attached to the seventh 

 cervical vertebra. It thus seems possible that this porpoise has found 

 it less difficult to begin a shifting forward of the thorax than further to 

 reduce the length of the cervical series as a whole. 



Certain additional reasons for cervical variation in the larger whales 

 may .also be considered, bearing in mind the thesis already explained 

 that in whales with heads of even moderate size the head alone furnishes 

 practically all of the prethoracic mass necessary for efficient locomotion ; 



FIGURE 21. Extremes of cervical vertebrae among toothed whales, illustrating 

 a series in which the seven components are separate, and one in which they 

 are all fused. Delphinapterus on left, Grampus on right. 



and this matter of head size is really more pertinent to the present chap- 

 ter than the last. 



One is so used to reading of the prodigeous size of the head in balaenid 

 whales that he is apt to take for granted that their heads are also of unu- 

 sual length. Because of the question of the intervertebral disks it is 

 difficult to find the precise proportions, but I have secured what measure- 

 ments I could from the literature, and these indicate not only that the 

 skull of Megaptera (25 to 31.4 per cent of the total length) may average 

 longer than in the balaenid whales (26.8 to 28.8 per cent) , but that there 

 is more individual than generic difference in Mysticeti. Rhachianectes 

 (22) and Balaenoptera acuto-rostrata (22.6 per cent) both have rather 

 short skulls and the others are intermediate. But there are important 

 differences elsewhere than in length of skull. In Balaenoptera, Sibbal- 



[149] 



