DIVISIONS OF CETACEA 



353 



form is characteristic of the genus. It is heart-shaped, more or 

 less, in Balaena, and somewhat cross- or T-shaped in the genus 

 Btilacnoptera. In the Odontocetes the ribs have, some of them, 

 the normal attachment by capitulum and tuberculum. In tlie 

 Mystacocetes the at- 

 tachment, where it 

 exists, is very loose, 

 and the tuberculum 

 alone is attached to 

 its vertebra. This 

 allows of the freer play 

 of the ribs during re- 

 spiration. The scapula 

 has a very character- 

 istic form in these 

 animals. The acro- 

 mion, where it exists, 

 is placed near the anterior margin of the shoulder blade, and 

 overlaps the generally long coracoid process. Clavicles are totally 

 absent. The pelvis is very rudimentary, consisting merely of a 

 single bonelet, to which are attached the rudiments (in some cases) 

 of a femur, and, in Balaena (Fig. 188), of a tibia also. 



Whales are to be divided into three great groups: — (1) the 

 Whalebone Whales or Mystacoceti ; (2) the Toothed Whales or 

 Odontoceti ; and (3) the entirely - extinct Archaeoceti or Zeu- 

 glodonts. 



Fig. 188. — Side view of bones of posterior extremity of 

 Greenland Kiglit Whale [Balaena niystice-tus). x ^. 

 i. Ischium ; /, femur ; i, accessory ossicle repre- 

 senting the tibia. (After Eschricht and Reinhardt) 

 (from Flower's Odcology.) 



Sub-Oedee 1. MYSTACOCETI. 



This division is thus characterised : — Teeth are never function- 

 ally developed ; they are present in the young, but replaced in 

 the adult by the baleen or whalebone ; the external respiratory 

 aperture is double ; the skull is perfectly symmetrical ; the rami 

 of the mandible are arched outwards and do not form a true 

 symphysis ; the sternum is always composed of a single piece of 

 bone ; the ribs articulate only with the transverse processes of 

 the vertebrae. 



The Mystacoceti are nearly invariably huge creatures, the 

 sole exceptions being the Pygmy Eight Whale, Neohalaena, and 

 VOL. X 2 a 



