BEAKED WHALES, FAMILY ZIPHIlDiE — TRUE. 73 



than the tenth pah-, 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 thoracic vertebra. 



In the adult male from Centerville beach, California, only ten pairs 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 end than those of the 

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

 and that this individual had thirteen lumbar vertebrse, 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 arc eleven thoracic vertebrcip, the trans- 

 verse processes of the eleventh bemg short and thick, 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 mmd that the number of thoracic vertebras in B. 

 iairdii is normally 11 and in B. arnuxii, 10. This would ordinarily be of little un- 

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

 number of thoracic and lumbar vertebrae in different individuals of the same species 

 is cormnonly met with. In the present family, however, the number of thoracic 

 vertebra? shows little variation, and as all known skeletons of B. Iairdii 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 diffei'- 

 ence in the form of the vertebrae themselves. As is well known, the transverse 

 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 vertebrpe by others at a lower level on the sides of 

 the centra. This change takes place differently and on difl'erent vertebra? in the 

 two species under consideration. 



VERTEBR.E. 



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

 frum for the articidation of the head of a ninth rib and no distinct transverse proc- 

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

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

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



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

 side of the centrmn, 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. (PI. 32, fig. 1.) 



In B. arnuxii the tenth thoracic is the second one having a distinct transverse 

 process, and the latter is broad distally and has the articular facet on the posterior 

 portion of the free margin. In B. Iairdii the tenth thoracic is the first having a dis- 

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



