BEAKED WHALES, FAMILY ZIPHIIDiE TRUE. 73 



than the tenth pair, and there can be no doubt that it really belongs to a terminal 

 pair. In this skeleton the transverse processes of the eleventh thoracic vertebra 

 are thick at the free end like those of the tenth tlioracic vertebra. 



In the atlult male from Centerville beach, California, only ten pahs of ribs are 

 present, but as the tenth is quite as long as the ninth, there is little doubt that an 

 eleventh pair was present originally. The eleventh thoracic vertebra, however, 

 has transverse processes longer and more flattened at the free cud than those of the 

 tenth thoracic. It is possible, of course, that the real eleventh thoracic is lacking, 

 and that this individual had thu-teen lumbar vertebra;, but of this there is no posi- 

 tive evidence. 



Only a few of the ribs accompany the skeleton of the adult female from St. George 

 Island, Alaska (Cat. No. 49726), but there are eleven thoracic vertebra, the trans- 

 verse processes of the eleventh bemg short and tliick, like those of the tenth, with a 

 distinct facet for the rib at the free end. This facet, however, is directed obliquely 

 backward and occupies only the posterior half of the free margm. 



There is no doubt in my mhid that the number of thoracic vertebrae in B. 

 hairdii is normally 11 and m B. armixii, 10. This would ordmarily be of little mi- 

 portance, as in nearly all kinds of cetaceans a variation of one, or even two, in the 

 niunber of thoracic ami hunbar vertebrae in different individuals of the same species 

 is commonly met with. In the present family, however, the mmiber of thoracic 

 vertebrw shows little variation, and as all knowai skeletons of B.hairdii have eleven 

 thoracics and all known skeletons of arnuxii appear to have ten thoracics, it seems 

 probable that this difference is specific. At all events, it is correlated with a differ- 

 ence in the form of the vertebra themselves. As is well known, the transvei-se 

 processes of the thoracics in this family undergo a sudden change of form and posi- 

 tion near the end of the series, the elevated processes on the anterior thoracics 

 being replaced on the posterior vertebra; by others at a lower level on the sides of 

 the centra. This change takes place differently and on different vertebra; m the 

 two species under consideration. 



VERTEBR.E. 



In B. arnuxii the eighth thoracic has no facet at the posterior end of the cen- 

 trum for the articulation of the head of a nintli rib and no distinct transverse proc- 

 ess, the tubercle of the rib articulating with a facet on the side of the metapophysis. 

 In B. bairdii the eighth thoracic is similar, but there is a distinct facet at the pos- 

 terior end of the centrum. (PI. 32, fig. 1.) 



In B. arnuxii the ninth thoracic has a very distmct transverse process on the 

 side of the centrum, while in B. hairdii the ninth thoracic has a short, slender process 

 attached to the side of the metapophysis and no facet at the posterior end of the 



centrum. (PL 32, fig. 1.) , . ,. • x . 



In B arnuxii the tenth thoracic is the second one havmg a distmct transverse 

 process and the latter is broad distallv and has the articular facet on the posterior 

 portion of the free margin. In B. bairdii the tenth thoracic is the first havmg a dis- 

 tmct transverse process on the side of the centrum. (PI. 32, fig. 1.) 



