26 



ZOOLOGY OF THE VOYAGE OF THE BEAGLE. 



skull of a Sumatran two-horned Rhinoceros, in the Museum of the College of 

 Surgeons (No. 816), the circumference of that part of the surface of the skull 

 which supported the posterior horn, and which includes precisely the same part 

 of the os frontis, presents the same character, the surface being broken by nume- 

 rous vascular impressions. On the supposition that this character of the supra- 

 orbitary arch in the Toxodon might indicate the superincumbency of a bony case, 

 I examined the skulls of two Armadillos, Dasypus Peba and Das. 6- ductus, and 

 found that in the Dasypus 6-cinctus, the supra-orbital ridges, which are slightly 

 elevated, to support the cephalic plate, presented, in a minor degree, a corres- 



v 



ponding rugosity. May we venture then to conjecture that the Toxodon was de- 

 fended by an ossified integument like the Armadillo, or that it was armed with an 

 epidermic production, analogous to the horn of the Rhinoceros ; or had the rugous 

 surface in question as little relation with the parts that covered it as the sculptured 

 surface of the malar bones in the Cavy ? 



After forming the rugged and prominent supra-orbital processes already 

 described, the frontal bone continues to send backwards a slightly elevated 

 ridge or crista, circumscribing the origin of the temporal muscles, but the extent 

 of this ridge, and the disposition of the inter-orbital portion of the frontal bones 



■ 



cannot be determined in the present mutilated specimen. The fractures it has 

 sustained are not, however, wholly unattended with advantage ; they expose the 

 structure of the diploe, which from its coarseness of texture and thickness, re- 

 sembles that of the Cetaceous crania ; and what is of still more importance, they 

 also demonstrate the existence and form of the frontal sinuses. 



The cavity of the nose is extensive, and the remains of the ossa spongiosa supe- 

 riora testify that the Toxodon enjoyed the sense of smell to a degree equal at 

 least to that of the Hippopotamus. 



The sphenoid bone resembles that of the Hippopotamus, but it contributes a 

 larger share to the formation of the internal pterygoid processes (p, PI. II.); these 

 are of a simple form, and more developed than in the Hippopotamus; they project 

 outwards to a greater extent, and terminate in a point. The sphenoid also sends off 

 a short and thick pointed process from the posterior part of the base of the internal 

 pterygoid processes. The ala of the sphenoid does not rise so far into the orbit, nor 

 does it articulate with the parietal bone, as in the Hippopotamus; but in this part of 

 its structure, is the same as in the Rhinoceros. The spheno-palatine foramen is 

 relatively larger than in the above-named Pachyderms, and is bounded above by 

 the descending orbital plate of the frontal bone. 



The palatal processes of the palatine bones terminate anteriorly between the 

 last molars, and extend backwards for some distance beyond the alveolar pro- 

 cesses, increasing the extent of the bony roof of the mouth posteriorly : this is a 

 structure in which the Toxodon deviates both from the Rodents, and Pachyderms, 



