Tursiops. It has no dorsal fin, the teeth are small and numerous (forty-four), and the pterygoids are separate. There are two species, T. borealis and T. peronii, the former being northern and the latter more widely spread.
The genus Cephalorhynchus has for its chief characters the following:—Teeth twenty-five to thirty-one, small and sharp. Pterygoids widely separated. Dorsal fin not falcate, but triangular or ovate in form. Beak not well marked off from the head. The species of this genus are all southern in range; four are perhaps to be allowed.
Fam. 3. Platanistidae.—This family of Odontocetes may be distinguished from the Dolphins by the following assemblage of structural features:—Cervical vertebrae all free, and each one of some length (for a Cetacean). Jaws long and narrow, with a considerable length of symphysis. Teeth very numerous.
This very meagre series of differential characters is largely due to Pontoporia on the Platanistid side, and to Monodon and Delphinapterus upon the Delphinid side. Otherwise the family Platanistidae would be extremely distinct. The two last-named genera have separate cervical vertebrae, and in the Beluga at any rate this is expressed externally by a quite distinct neck. Moreover, as Mr. True has pointed out, the pterygoid bones have not the involuted cavity below which characterises other Dolphins; and they have, what other Dolphins have not, an articulation outwards with the roofing bones of the skull. Sir W. Flower described the fact that in Inia (and the same occurs in Pontoporia) the palatines are separated from each other by the intervention of the vomer. In this feature they resemble certain Ziphioids, Berardius, Oulodon (= Mesoplodon) grayi, and Hyperoodon. The true Dolphins also appear to show the same intervention of the vomer in a few cases. There is nothing, therefore, distinctive from the Delphinidae in this feature.
The existence of cartilaginous sternal ribs in Inia and Platanista shows affinity between these two genera and the Physeteridae. Pontoporia is Dolphin-like in this particular, as it is also in the mode of articulation of the ribs with the vertebral column. But this last matter has already been dealt with. The principal reason for placing Pontoporia with the other two genera is the close resemblance which its skull bears to that of Inia.
The first genus of this family which will be noticed is Platanista.
