PHYSETERID^E 



255 



ramus of the mandible, variously situated, but generally at some 

 distance behind the apex (Fig. 86) ; its point directed upwards, and 

 often somewhat backwards, occasionally developed to a great size. 



FIG. 85. Mesoplodon bidens. From Reinhardt. 



Skull with the region around the nares as in Hyperoodon, except 

 that the nasals are narrow and more sunk between the upper ends 

 of the premaxillse ; like those of Hyperoodon, they are concave in 

 the middle line in front and above. No maxillary tuberosities. 

 Anteorbital notch not very distinct. Rostrum long, narrow, and 

 solid throughout. Mesethmoid in adult age ossified in its entire 



FIG. 86. Left lateral view of skull of Mesoplodon densirostris. 



length, coalescing with the surrounding bones, and showing as a 

 narrow band on the upper surface of the rostrum. Vertebrae : 

 C 7, D 10, L 10 or 11, C 19 or 20 ; total 46 to 48. Two or three 

 anterior cervicals united, the rest usually free. 



Though varying in form, the mandibular teeth of the different 

 members of this genus agree in their essential structure, having a 

 small and pointed enamel-covered crown, composed of true dentine, 

 which, instead of surmounting a root of the ordinary character, is 

 raised upon a solid mass of osteodentine. The continuous growth of 

 this greatly alters the form and general appearance of the organ 

 as age advances, as seen most strikingly in the case of M. layardi, 

 where the long, narrow, flat, strap-like teeth, curving inwards at 

 their extremities, actually meet over the rostrum, and must greatly 

 interfere with the movements of the jaw. In one species (M. grayi] 

 a row of minute, conical, pointed teeth, like those of ordinary 

 Dolphins, 17 to 19 in number, are present even in the adults, on 



