214 A BOOK OF WHALES 



t 



mandible at or near middle. Vertebral formula : C. 7 ; 

 D. 9 or 10; L. 10 or n ; Ca. 19 or 20-46 or 48. 

 Atlas and axis fused, sometimes also third. Sternum 

 of four or five pieces. Eight ribs two-headed. 

 Phalanges: I, i. II, 6. Ill, 6. IV, 3. V, 2. 



The most elaborate account of the skeleton of 

 Mesoplodon is contained in Sir W. Flower's descrip- 

 tion of the osteology of most of the species. The 

 skull agrees with that of Zip kins, and differs from 

 that of Hyperoodon and Berardiiis in the thorough 

 ossification of the mesethmoid, and its coalescence with 

 surrounding bones to form the very solid rostrum, 

 which in the adult has the density of ivory. The 

 tympanic bone of this genus differs from that of 

 Ziphius in having a well-marked groove at the pos- 

 terior end between the lobes. In this matter Meso- 

 plodon agrees with Berardius, and differs from 

 Hyperoodon, which in its turn agrees with Ziphius. 

 Ziphius and Hyperoodon are nearer in this particular 

 to Physeter, and the two other Ziphioid genera to the 

 dolphins. 



In these "beaked whales" the breadth of the base 

 of the rostrum and the relative positions of the two 

 foramina for the exit of the two branches of the second 

 division of the fifth nerve offer characters, which are 

 made use of, following Sir W. Flower, in the charac- 

 terisation of the species of the genus. The maxillae 

 have the characteristic ridges of the Ziphioid whales, 

 especially in M. hectori. The nasals are sunk be- 

 tween the extremities of the pre-maxillse. The. 



