FOSSIL MAMMALIA. 25 



part: the anterior transverse boundary is convex and smooth, and probably formed 

 part of the articulation for the lower jaw. The lower facet of the zygoma 

 anterior to the glenoid cavity gradually contracts in breadth, as it advances for- 

 ward, and at the distance of three inches from the articular cavity the zygoma 

 changes from a prismatic to a laminar form. It is at this point that the zygo- 

 matic suture commences, at the lower margin of the arch ; whence it extends 

 directly forwards for more than half its length, and then bends upwards at a right 

 angle. The zygomatic suture has a similar course in the Capybara, and Hippo- 

 potamus. 



The remainder of the zygoma is formed externally by the malar bone (g PI. II.), 

 which in its position is intermediate to the Rodent and Pachydermatous structures. 

 It is not suspended in the middle of the zygomatic arch, as in the former order; 

 neither does it extend into the region of the face so far anterior to the orbit as in the 

 Tapir or Hippopotamus. The exterior line of the malo-maxillary suture defines the 

 orbit anteriorly ; but from this line the maxillary bone extends backwards, along 

 the inner side of the malar portion of the zygoma, until it almost reaches the tem- 

 poro-malar suture ; thus abutting by an oblique surface against nearly the whole 

 internal facet of the malar bone, and materially contributing to the general 

 strength of the zygomatic arch. The malar bone is of considerable vertical 

 extent, and presents a rugged and thickened inferior margin for the attachment 

 of the masseter. The upper margin of the malar bone is smoothly rounded, and 

 presents a regular semi-circular excavation, forming the lower boundary of the orbit. 

 The relative magnitude of the zygomata to the entire cranium far exceeds in the 

 Toxodon that which exists in the Hippopotamus or any other known Pachyderm. 

 This arises from the great vertical development of the malar bone behind the orbit, 

 and the vertical expansion of the temporal portion of the arch. The oblique posi- 

 tion of the zygoma, descending as it advances forwards, is deserving of attention, as 

 the Toxodon, in deviating from the Pachyderms in these respects, makes an evident 

 approach to the herbivorous Cetaceans, as the Dugong and Manatee : in the 

 latter Cetacean we observe a similar development of the lower part of the zygo- 

 matic process of the malar bone. It is here, also, that we may perceive an indi- 

 cation of a resemblance between the Megatherium and Toxodon. 



There is no discernible trace of the lachrymal bone (e, PI. II.) having extended, 

 as in the Hippopotamus beyond the anterior boundary of the orbit : the lachrymal 

 foramen is situated rather deep in the orbit, and the bone itself appears to have 

 been of very small size. 



The surface of the supra-orbital process of the frontal bone(c, PI. II.) is de- 

 serving of attention, as it presents a peculiar ruggedness which is not found in any 

 other part of the skull ; the irregularity seems, as it were, to have been produced by 

 the impression of numerous small tortuous and anastomosing vessels. In the 



E 



