NO. 8 INTERRELATIONSHIPS OF THE CETACEA WINGE 43 



tween the nares and the occipital wall. The soft outer nasal passage is 

 pushed forward so that the blow-hole lies far to the front. The 

 adipose cushion has forced the roof of the braincase down by its 

 weight, and the whole braincase has sunk down below its original 

 level, so that the spinal marrow, or medulla oblongata, has to bend 

 down in an S-shaped curve to connect with the brain. The skull has 

 acquired a noticeably large size in proportion to the body, and most 

 of its bones have become conspicuously ponderous ; this is especially 

 noticeable as concerns the zygoma. Just as Physctcr is the largest 

 of all the toothed whales and pushes through the water with greater 

 force than any other, it is the one on which the resistance of the 

 water has had the most powerful influence. But it is a question 

 whether this high development is not a menace to the creature's life. 

 Irresistibly the water's pressure has caused the facial cushion to 

 grow to a disproportionate degree and in its turn to call forth a skull 

 the size of which is without relation to brain and body. The fate of 

 Physctcr, the most highly developed toothed whale, is much like that 

 of Balccna, the strangest whale-bone whale ; the difference is that the 

 pressure of the water in the one has acted most strongly on the outer 

 side of the head, in the other most strongly on the inner walls of the 

 mouth. Both animals are developed with such extravagant one- 

 sidedness that they appear to be in danger of certain extinction even 

 if their extirpation were not being worked at by man. 



Physeteridae.'' 



I. Occipital wall highly elevated in middle only, its position close 

 behind the nares. 



XiPHIINI. 



A. Dentition primitive : a long row of rather uniform 



small teeth in both upper and lower jaw. 

 Zygomatic process of the squamosal well 

 developed. 

 Argyrodelphini. 

 Argyrodclphis. 



B. Dentition atrophied : most of the teeth disappear, 



leaving one or two in each lower jaw specially 

 modified. Zygomatic process of the squamosal 

 somewhat reduced. 

 I. Longitudinal crest on maxillary above and in front 

 of orbit absent or slight. 



