The Delphini, properly so called, have also a narrow rostrum, but its length is scarcely three times that of the cranium ; the posterior extremities of the intermaxillary bones, toge ther with the maxillary and frontal bones, are raised, but not bent forwards ; the tempotal fossw in some species are as diminutive as in the Delphinorhynchi, but in others gradually recede from that character, and approach, by their expansion, to the form which they exhibit in the next generic type, viz. the Inia.
The cranium in this genus, besides the great extent of the temporal fossa, and the strong crista which forms its superior border, is also characterized by the shortness of the orbital fossa.
In the Phocana the rostrum is as remarkable for its breadth as it is in the Delphini for its narrowness ; this results from the great lateral development of the intermaxillary and max illary bones; but the antero-posterior extension of the bones is diminished, and the length of the rostrum does not exceed that of the cranium.
The Narwhals (1VIonodon) manifest their affinity to the Porpesses (Phocana) by the breadth and shortness of the rostrum, but differ from that and every other genus of Cetacea in the development of horizontal tusks in the inter maxillary bones, of which the left in the male and both in the female remain concealed in a rudimental state within the maxillary bones.
The cranium in the genus Hyperoodon, which includes the Great Bottle-noseWhale of Hunter, is at once distinguishable by the remarkable vertical crest which rises from the middle of the maxillary bones, the, contour of which pro cess descends suddenly behind, but extends more gradually and obliquely downwards an teriorly. The lower jaw in this genus has tvvo rudimental teeth at its anterior part.
Lastly, in the Gangetic Dolphin ( Plata nista) the cranium presents a marked resem blance to that of the Delphinorhynchus in the length and narrowness of the rostrum, and in the elevation and anterior curvature of its base; but on pursuing the comparison in detail, the structure and composition of this part of the skeleton presents several fundamental diffe rences, which at the same time indicate an affinity to the Cachalots ( Physeter). The most striking character in the cranium of the Platanista is presented by the maxillary bones, which, after having covered, as in the other Delphinide, the frontal bones as far as the temporal cristw, give off respectively a large osseous expansion, which arches forwards and forms a capacious vault above the spouting apparatus of the nostrils. In order to consti
tute this part, one of the processes inclines towards the other, so as almost to come in contact with it for the two anterior thirds ; but posteriorly they recede from one another to give passage to the blow-hole. The cavity beneath this singular bony pent-house is occupied by an interlacement of numerous osseous pro cesses, and by a close and hard fibrous sub stance.s If we suppose the cranium of a Dolphin to be proportionally very much shortened, the margins of the rostrum to be greatly expanded and raised, so as to render its superior surface concave ; the supra-frontal portions of the maxillary bones to be much developed and the margins extended upwards, thus form ing an immense basin, at the bottom of which lie the external orifices of the bony nostrils ; if also the occipital crest in the Dolphin vvere raised behind the maxillaries so as to aid them in the formation of the bony cavity, in the basis of which the parietals are almost con cealed, we should then have the skull of a Cachalot. The rostrum in the Catodonticke, not withstanding its immense size, is formed prin cipally by the maxillary bones, as the inter maxillaries and the vonier constitute a compa ratively small part of the intermediate portion. The nasal passages extend obliquely from below upwards and forwards, but are of very unequal dimensions, the one on the right side not having one-fourth the breadth of that on the left. A corresponding want of symmetry is shown in the nasal bones themselves, and the cranium generally; and this circumstance, it may be remarked, characterizes in a greater or less degree the skull in all the Zoophagous Cetacea.
The skull in the Whalebone-Whales ( Bala nide e ) is, however, the most symmetrical in its general form; it is characterized by the great relative predominance of the facial over the cranial portion, by the narrowness of the ros trum, and the curvature of the rami of the lower jai,v, which each extend outwards, in a convex sweep, far beyond the sides of the upper max illa, and converge to the symphysis, but with out meeting to form a bony union at their ante rior extremities.