form is characteristic of the genus. It is heart-shaped, more or less, in Balaena, and somewhat cross- or 
-shaped in the genus Balaenoptera. In the Odontocetes the ribs have, some of them, the normal attachment by capitulum and tuberculum. In the Mystacocetes the attachment, 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 respiration. The scapula has a very characteristic form in these animals. The acromion, 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.
Fig. 188.—Side view of bones of posterior extremity of Greenland Right Whale (Balaena mysticetus). × ⅛. i, Ischium; f, femur; t, accessory ossicle representing the tibia. (After Eschricht and Reinhardt) (from Flower's Osteology.)
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 Zeuglodonts.
Sub-Order 1. MYSTACOCETI.
This division is thus characterised:—Teeth are never functionally 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
