AQUATIC MAMMALS 



sorts and shortened in others. Neobalaena has no less than 17 pairs of 

 ribs, while Hyperoodon has 9, which is the least number in any living 

 mammal. Other sorts vary between these two extremes. There is in- 

 dicated some instability in the anterior thorax by the fact that a few 

 cetaceans have a pair of rudimentary cervical ribs, and that occasionally 

 in mysticetes the first rib is bifurcated, one head going to the seventh 

 cervical and the other to the first thoracic vertebra. It is perhaps usually 

 stated that some whales exhibit this condition, which gives the impres- 

 sion that it is a character of certain species. As far as I can ascertain, 

 however, it is an individual though perhaps fairly common develop- 

 ment. Thus no mounted mysticete skeleton in the National collection 

 shows any bifurcation of the first rib, while the Sei whale obtained in 

 Japan by R. C. Andrews (1916) does show it. In view of the fact that 

 the more primitive fossil whales all had seven cervical vertebrae, so far 

 as known, the two details mentioned above might well be interpreted as 

 an indication of morphological effort to attain, in a somewhat different 

 manner, a forward shift of the thorax. But against this reasoning there 

 is the totally antagonistic fact that Neobalaena, anomalous in so many 

 ways, has the first eight vertebrae without ribs, so that in this genus there 

 seems to have been a shift of the thorax toward the rear. 



The thorax of whales varies in the strength with which the ribs are 

 attached to the vertebral column. Ribs with both tubercular and capitu- 

 lar attachment vary in number among toothed whales from at least eight 

 (and possibly more in some sorts not examined) in Berardius, Kogia, 

 Monodon, and Phocaena to as few as four in Stenodelphis and some in- 

 dividuals of Tursiops. It has been stated in the literature that the ribs 

 of existing mysticetes are all single-headed, but this is erroneous. Condi- 

 tions are variable and the articulations of the ribs are certainly reduced, 

 but all individuals of this group which I have examined show at least a 

 few ribs that cannot be called single-headed, possibly with the exception 

 of Megaptera. It seems normal for the first rib to have no capitular pro- 

 jection toward the centrum. In Eubalaena this is also lacking from the 

 second rib, it is slight upon the third, and upon the fourth reaches half 

 way from the tuberculum to the centrum, thence gradually shortening in 

 caudal sequence. In Rhachianectes the second to sixth ribs inclusive have 

 capitular projections that almost reach the centra. In Sibbaldus the sec- 

 ond, third and fourth ribs show this character, while in Megaptera there 

 is some capitular projection upon the third rib, which, however, falls 

 considerably short of the centrum, and this is still less defined in the more 

 posterior ribs. Thus, although there may not be bony connection between 



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