Peterson: The Osteology of Promerycochcerus. 197 



and rugose zygomatic arch with its posterior vertical plate, which is 

 high and broad, extending backward nearly on an even transverse 

 line with the posterior face of the occipital and mastoid plates, and 

 we have warrant for the musculature adumbrated in these sketches in 

 clay. It has been suggested by Dr. Holland in conversation that the 

 animals might have had warty protuberances covering the superior 

 and outer portions of the zygomatic arches. It seems, however, to 

 the writer, more probable that the rugose zygomatic arch was intended 

 for muscular attachments over which the epidermis was drawn as 

 represented in the models. ^^ It has already been stated that the 

 mandible is proportionally long, deep, and heavy as in the pigs, and 

 that the masseter muscle did not extend forward on the jugal and the 

 mandible, as on some recent ungulates with long jaws and large teeth 

 (Eqiins cahallus); hence it would appear that the broad, heavy, and 

 rugose zygomatic arch and the deep excavation on the inner face of 

 the angle of the lower jaw were for the attachment of muscles sufifi- 

 ciently powerful to properly swing the heavy jaws, which are full of 

 robust teeth. The position of the ear is well up on the side of the head, 



1^ The opinion expressed by Mr. Peterson is one from which I am not at all 

 indined to dissent, and I think it is in the main absolutely correct. There is not 

 a shadow of doubt that the enlargement of the zygomatic arch had as its primary 

 object the supply of surfaces sufficiently large for the attachment of the powerful 

 muscles which were required in order to the mastication of coarse and rough food, 

 upon which these creatures probably fed. Nevertheless the development of the 

 zygoma in this family is so remarkable, and so entirely different from that in other 

 families in which an equal or even greater muscular power is called for, as to suggest 

 that in addition to its use as a support for heavy musculature, it may indicate 

 some external embellishment of the features, which may have been of use to these 

 animals. In the mandril and some of the allied apes we have a thickening of the 

 bones of the face, which especially in the mandril are covered with highly pigmented 

 skin; in the wart-hog of Africa the excrescences, which impart such a hideous 

 appearance to the animal, are coupled with corresponding thickenings of the sub- 

 jacent osseous tissue. It does not appear necessary to suppose that the rounded 

 and widened upper and outer surface of the zygoma was required to furnish attach- 

 ments for the origin of the masseteric muscle in Promerycochcerus and its allies; it 

 is more probable that these muscles had their attachments on the inner and lower 

 surface of the widely expanded bone. While I do not strenuously advocate the 

 view, which I proposed to Mr. Peterson in conversation, and in fact when helping to 

 model the heads did not attempt to express it in the clay, it nevertheless seems to me 

 to be worthy of consideration and highly plausible. No scheme of mere muscula- 

 ture applicable to the case seems to call of necessity for such a peculiar develop- 

 ment of the zygomatic arch, especially on its upper and external surfaces, as is seen 

 in some species of this family. W. J. Holland. 



