3 So 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, 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 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 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 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 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 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 Inia. 



The first genus of this family which will be noticed is Platanista. 



