VII!.] CETACEA. 



93 



is their condition in the foetus of BahnJia mysticdus^ as 

 described by Eschricht and Reinhardt. 



Balcsnoptera rostrata^ the smallest of the "Whalebone 

 Whales, has but ii pairs of ribs, Megaptera lojigimana 14 

 pairs, the Greenland Right Whale {Balcena mysticetus) 

 usually 13, and, the larger Fin Wliales {BalcEJioptem 7niisculus 

 and sibbaldii) 15, and occasionally 16, the highest number 

 known in any Cetacean. In these last, it not unfrequently 

 happens that the hindermost rib, having only the middle 

 or lower portion developed, is separated by a wide interval 

 from the vertebral column, a very rare condition, as in most 

 otlier cases where the hinder ribs are rudimentary the part 

 in immediate connection with the vertebra remains. 



The first rib presents a very anomalous condition hi some 

 Whalebone Whales, being apparently double, probably owing 

 to the coalescence of a supplemental cervical rib with the 

 ordinary first thoracic rib. In some spi^cies (as Balcenoptera 

 laticeps) this appears to be of constant occurrence ; in others, 

 it is occasional.^ 



In tlie Odontoceti or Toothed-whales, as the common Dol- 

 phin and Porpoise, the ribs (usually 12 or 13 pairs) are long 

 and slender. The first four or five have tubercles, by which 

 they articulate with the transverse process of the thoracic 

 vertebrae, and long necks and heads, reaching to the side of 

 the antecedent vertebra, near the junction of the body and 

 the arch (see Fig. 20, p. 53). The posterior ribs, however, 

 lose the neck, and are solely articulated by the tubercle to 

 the transverse process. There are usually 7 pairs of rather 

 short, straight, but strongly ossified sternal ribs, and often 

 small intermediate ribs, sometimes distinctly ossified. 



In the aberrant Physeteridce^ including the Sperm Whale, 



^ See Professor Turner " On the so-called Two-headed Ribs in Whales 

 and inJNIan." {Journal of Anatomy and Physiology, May 1S71.) 



