PHYSETERIDE 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. 
Fic, 85.—Mesoplodon didens. From Reinharat. 
Skull with the region around the nares as in Hyperoddon, except 
that the nasals are narrow and more sunk between the upper ends 
of the premaxille; like those of Hyperoddon, 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 
Fic. $6.—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. V ertebree : 
C7, D10,L10 or 11, C19 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 i in the case of JI. 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 (1/. vvuyi) 
a row of minute, conical, pointed teeth, like those of ordinary 
Dolphins, 17 to 19 in number, are present eyen in the adults, on 
