566 



CETACEA. 



transverse superior cranial or occipital ridge, 

 and the superior maxillary bones approach very 

 close to the same part, the frontal bone seems 

 to be represented by a very narrow osseous 

 band traversing the cranium from right to left, 

 and dilating at each extremity to form the roof 

 of the orbit (c, c ). But when the maxillary 

 bones which have extended over the whole 

 anterior part of the cranium are raised, the 

 frontal bone is then seen to be of much larger 

 size than the external appearances indicate. 



The two nasal bones (d, d) are in the form 

 of oblong rounded tubercles, set deeply in 

 two depressions in the middle of the frontal 

 bone, and in front of which the nasal passages 

 (e, e) are continued vertically downwards. 

 The two intermaxillaries (f,f) form the exter- 

 nal and anterior margin of the nasal apertures. 

 The cribriform plate of the ethmoid consti- 

 tutes the posterior wall of the nasal passages ; 

 and in this plate there are three or four small 

 perforations. The remainder of the circum- 

 ference of the bony nostrils is formed by the 

 maxillary bones, of which a small part appears 

 at g : their septum is the vomer, which is 

 joined to the ethmoid as usual. 



The malar bone is an irregular flattened bone, 

 which assists the frontal in forming the orbit, 

 and, like it, is covered by the maxillary bone : 

 it sends backwards a long and slender process, 

 which articulates with the zygomatic process 

 of the temporal bone, and forms the only 

 bony boundary of the lower part of the orbit. 

 The zygomatic process of the temporal bone is 

 united to the post-orbital process of the frontal, 

 bounding the orbit posteriorly; and thus the 

 zygomatic arch is exclusively formed by the 

 temporal bone : this bone terminates at the 

 temporal ridge, having but a small extent of 

 development on the side of the cranium, and 

 not entering at all into the composition of the 

 posterior convex surface. At the base of the 

 cranium the basilar and the lateral occipitals 

 develop expanded plates, which join the ptery- 

 goideal alae of the sphenoid, and a lamina of 

 the temporal bone, to which the petrous and 

 tympanic bones have a ligamentous attach- 

 ment. The parietal bones also extend behind 

 the temporals, to aid in completing the basilar 

 walls of the cranial cavity, so that the temporal 

 bone is almost excluded from entering into the 

 composition of the cranium, serving merely to 

 close some small vacancies left by the parietals : 

 this structure is of great interest, as we perceive 

 in it the commencement of that displacement 

 of the temporal bones from the cranial parietes 

 which is characteristic of the small-brained and 

 cold-blooded classes of Vertebrata. 



The differences between the Dugong and 

 Manatee in respect to the structure of the 

 cranium, we have seen to resolve themselves 

 almost entirely into the expansion and elonga- 

 tion of the intermaxillary bones in relation to 

 the tusks, which they are destined to support in 

 the former animal; and we shall find on a com- 

 parison of the skulls of the Delphinid& toge- 

 ther, that they also differ from one another, 

 chiefly in the forms and proportions of their 

 maxillary and intermaxillary bones. 



The Delphinorhynchi are characterized, first, 

 by an extremely narrow rostrum, the length of 

 which is four times greater than that of the 

 cranium ; secondly, by the anterior curvature 

 of the posterior extremities of the intermaxil- 

 laries, which, as it were, draw forwards in the 

 same direction the maxillary, the frontal, and 

 even the occipital bones ; thirdly, by the posi- 

 tion of the nasal bones, which are sunk in 

 between the frontals and intermaxillaries ; 

 fourthly, by the very diminutive size of the 

 temporal fossce. 



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 tempoial 

 fossae in some species are as diminutive as in 

 the Detphinorhynchi, 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 Phoc&niz 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 (Monodon) 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 Boltle-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 two 

 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 

 Delphinidd, the frontal bones as far as the 

 temporal cristae, give off respectively a large 

 osseous expansion, which arches forwards and 

 forms a capacious vault above the spouting 



