SKELETON OF CETACEA. 



41^ 



283 



Anchjiosed mass of cervicals, 

 Balcena ynysticetus. 



is developed, as a rule, and the head of the first dorsal pleurapo- 

 physis abuts against its centrum. The 

 diapophyses progressively elongate in 

 the dorsals, and support the corre- 

 spondingly lengthening ribs. The 

 discoid terminal epiphyses long retain 

 their individuality in Cetacean verte- 

 brae, longest in Balcena.^ 



The ^ breast-bone ' in PhyseteridcR 

 consists usually of three sternebers, 

 each ossified from a pair of centres 

 which tardily coalesce, save in the first 

 and largest. Four pairs of ribs directly 

 unite with this sternum, as in Delphinus 

 Tursio, in which the sternebers ultimately coalesce into a single 

 bone. In Hyperoodon and ZipMus there are four sternebers, with 

 a vacuity at the middle of each articulation, and five pairs of ribs 

 articulate with the sternum. The sternum in Whales consists of 

 but one bone, to which is usually connected a single pair of ribs. 

 The articular surfaces for these mark its sides : in the more active 

 Balcenoptera the bone is deeply notched in front, produced behind, 

 fig. 284, where it is ridged below. The sternum 

 is short and broad, shield-shaped, in Balcena : 

 rhomboid, sometimes with a central perfora- 

 tion, in KyphohalcBna, Esch. One or two of 

 the posterior pleurapophyses are loosely sus- 

 pended by ligament to the diapophyses of their 

 vertebra in many DelpMnidcB? 



B. Skull. — The cranial neural arches con- 

 tinue to manifest the peculiar proportions which are shown in an 

 exaggerated degree in the cervical series. In an advanced foetal 

 Cachalot {Physeter macrocephalus) I find the elements of the 

 epencephalic arch unanchylosed. The lateral margins of the 

 anterior half of the basioccipital are produced and bent obliquely 

 downward. The exoccipitals are much produced and expanded 

 laterally, like the neurapophyses of the atlas in fig. 283, i : they 

 are deeply notched below. The superoccipital contributes the 

 upper ends of both condyles ; it is in the form of a vertical plate, 

 convex from side to side, and developes internally a falciform 

 crest. The superoccipital is overlapped at its lower and lateral 



* XL7V., vol. ii, p. 440. In Fin-Whales the anchylosis is noted in certain vertebra) 

 of no. 2444, p. 441. 



- xcviir. p. 8. xcxx'., p. 72, taf. 4 .and 5. 



£ E 2 



284 



Sternum of Balwnoptera. 



