THE WHALEBONE WHALES OF THE WESTERN NORTH ATLANTIC. 



139 



The European specimens show a substantial agreement in these characters 

 with the American specimens and with each other, but in the case of the Borselaer 

 skeleton, as reported by Van Beneden (4), the first vertebra with perforated 

 transverse process is much farther back in the series than in other specimens. This 

 striking peculiarity may perhaps be safely regarded as an individual variation. 



CHEVRON BONES. 



In Baloenoptera pTiysalus the series of chevrons begins anteriorly with a small 

 bone, followed by a very large one, after which the bones decrease gradually in 

 size to the posterior end of the series. Van Beneden made the following signifi- 

 cant remarks in connection with the Borselaer skeleton : " The chevron bones are 

 21 in number; the last three of the caudal vertebrae are alone without them. We 

 count among these bones the osseous rudiments visible in the cartilages, and which 

 are very rarely preserved. Without particular attention, we should not have found 

 in all but 15 of these bones" (4, 24.) There is little doubt that the series found 

 in the majority of specimens in museums is incomplete, and the variations cannot, 

 therefore, be relied upon in investigations of this kind. The numbers recorded in 

 various European and American specimens are as follows : 



BAL^ENOPTERA PHYSALUS (L.). EUROPEAN AND AMERICAN. CHEVRONS. 



STERNUM. 



In Flower's diagnosis quoted above (p. 131) the sternum is thus referred to : 

 " Sternum broader than long, in the form of a short, broad cross, of which the pos- 

 terior arm is very narrow ; it might perhaps be compared to the heraldic trefoil ; it 

 is subject, however, to considerable individual variation." 



In comparing figures of the sternum of European specimens, the variation at 

 first appears excessive, but one soon perceives that much of it is due to differences 

 in age. The figures brought together on pp. 140 and 141 show the sternum of 

 various European and American specimens. (See text figs. 8 to 32.) 



In the midst of this wide variation the sternum of immature individuals takes 

 quite uniformly the form of a trefoil with short stem and wings, and deeply emar- 

 ginate anterior border, as shown in the St. Vigor, Lofoten Ids., and Brussels Museum 

 specimens. It also occurs in the National Museum specimens Nos. 16039 and 



1 "The chevron bones appear to be all present" (Flower). 



1 Thirteen well developed, the first small, the last two cartilaginous. 



1 The first large and hence probably preceded originally by a smaller one. 



