AQUATIC MAMMALS 



The Mysticeti never pursue an individual food item but whole schools 

 of relatively small fish or shrimp. Hence there should be no need 

 for any motion of the head connected solely with feeding save that of 

 opening the mouth. There is thus need only for such slight movements 

 as are involved in swimming and steering — very simple ones which have 

 allowed the occipital musculature to assume a correspondingly simple 

 form. They are unlike Odontoceti also in that the rostrum is not stati- 

 cally held in a depressed posture. In balaenopterids the whole head 

 anterior to the occipital plane presents dorsally a practically straight line. 

 In other sorts the whole dorsum of the skull is curved, but still the "aver- 

 age" rostral axis seems fairly on a line with that of the body. At any 

 rate all mysticetes hold the head in a more elevated position than do 

 generalized odontocetes. 



In connection with the occipital tilt of mysticetes Miller (1923) con- 

 sidered as essential the presence of a downward pulling force applied 

 to the head. But the downward force, furnished by water pressure, 

 against the head of a baleen whale could hardly be relatively greater than 

 that experienced by a pig while rooting. A force is present, certainly, 

 but the critical factor is the position in which the head is normally held 

 while the force is applied, rather than the force itself. If a whale habitu- 

 ally held its head at a right angle to the neck it would not have a forward 

 tilt to the occipital, but a backward one. A better understanding of the 

 situation may be obtained by examining occipital details in relation with 

 external details. It will be found that in Rhachianectes, the most primi- 

 tive of living mysticetes, the occipital tilting, while perhaps more pro- 

 nounced than in odontocetes, is very moderate: at the same time it has 

 the longest neck. Tilting is most accentuated in the right whale (ba- 

 laenid) type, and intermediate in other sorts {Sibbaldus, Balaenoptera, 

 Megaptera) . Some qualification is necessary in the statement regarding 

 balaenid whales, however. In these the occiput is most sharply tilted in 

 relation to the cranium proper, but the entire skull is sharply curved and 

 sickle-shaped (fig. 20), and the head so held in normal posture that 

 the occipital tilt in relation to body axis is only moderate, and actually 

 less than in balaenopterids. In reality occipital conditions in living mysti- 

 cetes may be divided into two categories. In one, comprising Sibbaldus, 

 Balaenoptera and Megaptera, the occipital tilt to body axis is extreme, 

 and may be referred to as the balaenopterid type. In the other group, 

 consisting of the gray and balaenid whales, it is more moderate, and may 

 be called the balaenid type. The rostra of the latter are always down- 

 wardly curved (see fig. 9) in varying degree, and the conformation of 



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