The skull of a toothed whale in general appearance is quite unlike that of a whalebone whale; the nasals are very small, and the maxillæ, premaxillæ and frontals meet above the nostrils to form a bony ridge which is sometimes developed into an extraordinary crest. In some cases the crest overhangs the blowholes and is asymmetrical, the right side being much more strongly developed than the left.
The facial portion, or rostrum, of the right whales is narrow and greatly arched, but in the Balænopteras it is wide and flat; in the toothed whales it may be either wide and concave, as in the Physeteridæ, or narrow and beak-like as in the Ziphiidæ and Delphinidæ.
The neck, or cervical, vertebræ of all the Cetacea are exceedingly thin and plate-like and usually either two or three of the entire series are fused. In large-headed species, such as the right whales, the neck is reduced to a minimum and the cervical vertebræ are all joined in a solid mass to bear the weight of the enormous skull.
The remainder of the spinal column, as in all mammals, is divided into dorsal, lumbar and caudal vertebræ. The first series bear ribs and the last, which are those of the “tail region,” may be distinguished by the V-shaped “chevron bones” attached to the lower side of each vertebral body; because of the absence of functional hind-limbs no sacrum is present.
The ribs of the whalebone whales differ from those of other mammals because all but the first two or three have lost the capitulum, or head, and articulate by only the tuberculum to the transverse processes of the vertebræ.
In the baleen whales the sternum, or breast bone, is so reduced that it only articulates with the first pair of ribs, the lower ends of those remaining being free. Thus with the weak attachment of the ribs to the vertebræ and no fastening to the sternum, a loose “thoracic box” is formed, which is capable of great lateral movement as the enormous lungs expand and contract.
In the toothed whales conditions are somewhat different. Many of the ribs have the normal attachment by head and tubercle to the vertebræ and are joined by their lower ends to the sternum, which consists of several pieces; thus the thoracic box is much more rigid than in the baleen whales.
The bones of the fore-limbs of ordinary mammals are present in the cetacean flipper, but they become greatly flattened and overlaid with adipose tissue to form a paddle. In the right whales the five fingers of the mammalian hand are present, but in others one finger has been lost, and the digits are greatly elongated. The scapula, or shoulder blade, is a wide, flat, fan-shaped bone, and the clavicles, or collarbones, have entirely disappeared. The hind-limbs are rudimentary, when present at all, only being represented by bony nodules, and the pelvis is reduced to two spindle-shaped bones quite unlike that of land mammals.
The skeleton of each group of the Cetacea, although similar in general characters, varies enormously in the details of construction, and to anyone interested in osteology will prove a fascinating subject for investigation.