XII DIVISIONS OF CETACEA 2i59 
form is characteristic of the genus. It is heart-shaped, more or 
less, in Balaena, and somewhat cross- or T-shaped in the genus 
Lalaenoptera. In the Odontocetes the ribs have, some of them, 
the normal attachment by capitulum and tuberculum. In the 
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 
: hs ton. FG. 188.—Side view of bones of posterior extremity of 
has a very character- Greenland Right Whale (Balaena mysticetus). x 4. 
istic form in_ these z, Ischium ; 7, femur; ¢, accessory ossicle repre- 
. l T1 senting the tibia. (After Eschricht and Reinhardt) 
animals. 1€ acro- (from Flower’s Osteology.) 
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. 
Sus-OrDER 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 Right Whale, Neobalaena, and 
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