252 CETACEA 



or in small herds. By their dental and osteological characters they 

 are easily separated into four distinct genera. 



Hyperobdon. 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 premaxillae 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 premaxillse, 

 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 

 maxillffi 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. 

 Vertebra: C 7, D 9, L 10, C 19 ; total 45. All the cervical 

 vertebra? united. Upper surface of the head in front of the blow- 



Fig. 84.— Hyperoodon rostratus. From a female specimen taken off the coast of Scotland, 1SS2. 



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. 2 So different, indeed, is the appearance of 

 the skull of an old male from that of a female individual that 



1 Lacepkle, "Table des Ordres," Hid. Nat. des Citads, p. xliv. (1804). 

 2 See the figures in the Proc. Zool. Soc. 1882, pp. 728, 729. 



