22 



ZOOLOGY OF THE VOYAGE OF THE BEAGLE 



mechanical instrument, subservient to the function of digestion ; we next proceed 

 to consider the structure and composition of those cavities of the skull which gave 

 lodgment and protection to the organs of special sense, and endeavour to deduce 

 from their structure conclusions as to the degree in which the organs were deve- 

 loped, and the circumstances under which the senses were exercised. 



The orbit of Toxodon forms the anterior boundary of the zygomatic area ; 

 it is about as distinctly defined as in the Tapir or Dugong, having its osseous rim 

 less complete than in the Hippopotamus, yet more developed than in the Capy- 

 bara, Coypus, and many other Rodentia, in which the orbit is scarcely distinguish- 

 able in the cranium from the small space occupied by the origin of the temporal 

 muscle. 



The lower boundary of the orbit in Toxodon is formed by an excavation in 

 the upper and anterior part of the zygoma ; the upper boundary by a strong and 

 rugged overarching process of the frontal bone, the posterior angle of which 

 (a, PI. III.) descends a little way, but leaves a space of three inches and a half be- 

 tween it and the opposite angle of the malar bone below (b, PI. II. and III.), the 

 circumference of the orbit being completed probably by ligament in the recent sub- 

 ject. The cavity thus circumscribed is remarkable for the preponderance of the 



V 



vertical over the transverse or longitudinal diameter, and indicates great extent of 

 motion of the eyeball in the vertical direction, such as may be supposed to be well 

 adapted to the exigencies of an amphibious quadruped. The orbit of the Capybara, 

 or Water-hog, makes a near approach to the form just described. In the elevation 

 of the supra-orbital boundary, and its outward projection in the Toxodon, we per- 

 ceive an approximation to the form of the orbit in the Hippopotamus, but the size 

 of the orbit is relatively larger in the Toxodon, which in this respect manifests 

 its affinity to the Rodentia. 



In that part of the bony structure of the auditory apparatus, which is visible on 

 the exterior of the cranium, the skull of the Toxodon presents a character in which 

 it recedes from the Rodentia. In these, the tympanic portion of the temporal bone 

 is remarkably developed, forming a large bulla ossea between the glenoid cavity and 

 the occiput; and it always remains disunited to the other elements of the temporal 

 bone. In the Toxodon the tympanic bone (<?, PL II.) consists of a rough com- 

 pressed vertical osseous plate, wedged in transversely between the occiput and the 

 posterior part of the glenoid cavity. The internal extremity of this plate points 

 inwards and forwards, representing the styloid process ; behind this is seen the 

 petrous bone, which forms a small angular protuberance at the basis cranii, and is 

 less developed than in the Hippopotamus. Anterior to the petrous bone are the 

 orifices of the Eustachian tube, and carotid canal ; external to it is the great 

 foramen lacerum, for the jugular vein and nervus vagus; and behind it is the 

 anterior condyloid foramen. The foramen auditorium externum is only half an 



