380 THE SUSU CHAP. 
Tursiops. It has no dorsal fin, the teeth are small and numerous 
(forty-four), and the pterygoids are separate. There are two 
species, 7. borealis and 7. peronit, the former being northern and 
the latter more widely spread. 
The genus Cephalorhynchus has for 1ts chief characters the 
following :—Teeth twenty-five to thirty-one, small and_ sharp. 
Pterygoids widely separated. Dorsal fin not falcate, but tri- 
angular 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 /nia (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 Hypero- 
odon. The true Dolphins also appear to show the same inter- 
vention 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 Jnia 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 verte- 
bral 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 Znia. 
The first genus of this family which will be noticed is Platanista. 
