252 CETACEA 
or in small herds. By their dental and osteological characters they 
are easily separated into four distinct genera. 
Hyperotdon.1—A_ small conical pointed tooth at the apex of each 
ramus of the mandible, concealed by the gum during life. Skull 
with the upper ends of the premaxille rising suddenly behind the 
nares to the vertex and expanded laterally, their outer edges 
curving backwards and their anterior surfaces arching forwards and 
overhanging the nares; the right larger than the left. Nasal bones 
lying in the hollow between the upper extremities of the premaxille, 
strongly concave in the middle line and in front; their outer edges, 
especially on the right side, expanded over the front of the inner 
border of the maxilla) Very high longitudinal crests on the 
maxille at the base of the rostrum, extending backwards almost to 
the nares, approaching each other in the middle line above; some- 
times so massive that their inner edges come almost in contact. 
Anteorbital notch distinct. Mesethmoid but slightly ossified. 
Vertebre: C 7, D 9, L 10, C 19; total 45. All the cervical 
vertebree united. Upper surface of the head in front of the blow- 
Fic. 84.—Hyperoddon rostratus. From a female specimen taken off the coast of Scotland, 1882. 
hole very prominent and rounded, rising abruptly from above the 
small, distinct snout. 
The genus is known typically by H. rostratus (Fig. 84), but an 
imperfect skull has been made the type of H. planifrons—a species 
differing considerably in cranial characters from the typical one. 
The females and young males of the first-named species have the 
contour of the head of the same general form as in Fig. 84; the 
premaxillary crests of the cranium being widely separated from 
one another, and terminating in comparatively sharp edges. In the 
males, however, as age advances the summits of these crests become 
gradually expanded and flattened, till they are almost or quite in 
contact in the middle line. This development of the maxillary 
crests produces a corresponding elevation and flattening of the front 
of the head, so that in very old males this aspect presents a flattened 
disc -like surface rising abruptly from the beak (which thus 
becomes almost buried) and situated in a plane nearly at right angles 
to the line of the back.” So different, indeed, is the appearance of 
the skull of an old male from that of a female individual that 
1 Lacépéede, ‘Table des Ordres,” Hist. Nut. des Cetacés, p. xliv. (1804). 
* See the figures in the Proc. Zool. Soc. 1882, pp. 728, 729. 
