24 



ZOOLOGY OF THE VOYAGE OF THE BEAGLE. 





condyles, but the entire circumference of the occipital foramen are visible. (See 

 PL III.) The upper part of the supra-occipital plate presents a broad rugous 

 depression, indicative of the insertion of strong cervical muscles, and probably of 



a ligamentum Nucha* 



The ex-occipital processes advance forwards for about an inch beyond the 

 condyles, and then suddenly extend outwards at right angles to the former line, and 

 terminate in the form of vertically compressed bony plates ; the lower rugged margins 

 of which represent or perform the office of the mastoid processes (d, d, Pis. II. and 

 III.). The breadth of the entire occipital region of the skull (fig. 1, PL IV.) 

 appears to have been, allowing for the fractures, about one-third more than the 



height of the same part. 



The great development of the tympanic bones in the Rodentia, occasions the 

 intervention of a considerable space between the occipital bone and the zygo- 

 matic process of the temporal ; but in the great Toxodon, in which the sense of 

 hearing was doubtless inferior to that enjoyed by the small and timorous Rodents, 

 the tympanic bone is reduced to a thin plate, which is wedged in between the oc- 

 ciput and glenoid cavity. In this structure, and the consequent posterior position 

 of the glenoid cavity, there is a close resemblance between the Toxodon and the 

 Hippopotamus, Tapir, and Rhinoceros. 



The squamous element of the temporal hone (n, PI. II.) forms a small propor- 

 tion of the lateral walls of the cranium, and also enters into the composition of the 

 lateral and superior parts of the posterior region of the cranium, where two deep 

 fossa? perforated by large vascular foramina, indicate the junction of the squamous 

 bones with the supra-occipital bone. The posterior surface of the skull is thus 

 divided into three broad and shallow depressions, the two lateral facets being 

 slightly over-lapped by the middle one, at their junction with it. In this structure 

 the Toxodon rsembles the Hippopotamus, and differs considerably from the Ce- 

 tacea, in which the occipital region is rendered convex by the extraordinary de- 

 velopment of the brain within. 



The zygomatic process of the temporal bone projects boldly outwards at its 

 commencement, where it is of great strength, and three-sided ; the glenoid cavity 

 extends transversely across the base or inferior surface of this part ; the lateral 

 surfaces converge to form the ridge or upper boundary of the zygoma. The 



depth of the glenoid cavity is increased by a transverse production of bone both 



before and behind it: the posterior process (g, PI. II.) descends the lowest, and 

 affords the requisite defence against backward dislocation of the lower jaw ; the 

 pressure of the condyle against this process is denoted by a well defined, transversely- 

 ovate, flattened and smooth surface, as if the bone had been planed down at that 



* I have ascertained that this elastic ligament exists in the neck of the Dugong. 



