310 MEMOIRS OF THE CARNEGIE MUSEUM 
Anteriorly it apparently narrows rapidly, and there is a small rough tuberosity with 
a narrow groove through it in the median line. The basi- and presphenoids eyvi- 
dently have much less lateral extent than in recent genera, as the space between 
the tympanic regions is quite narrow. The foramina opticum and rotundum are 
close together ; the latter is quite large. Inferiorly the sphenoids send out strong 
wing-shaped lamine. These alee are firmly joined to the squamosal at the internal 
base of the glenoid cavity and extend forward and downward, and obliquely out- 
ward, to again form a strong contact with the posterior part of the maxillary and 
the pyramidal process of the palatine. Some of the John Day species have a sim- 
ilar structure in this region according to Cope.* The region of the posterior nares 
of Thinohyus siowxensis is one of the chief characteristics of the species. The pos- 
terior exit of the orifice is between the anterior part of the tympanic bulle, and its 
almost vertical direction is indicated by the large swelling on the sphenoids at the 
posterior boundary of the orbital fossa. In the peccary and Platygonus leptorhinus 
this orifice is located well posteriorly, but not so far back as in Thinohyus siouxensis. 
The posterior narial border is divided anteriorly into three deep triangular 
grooves ; one in the middle and one on either side of an osseous septum. The outside 
grooves are bounded externally by the deep wall of the inferior alee of the sphenoids 
referred to above. ‘The foramen ovale and lacerum anterius are located at the an- 
terior base of the tympanic bulla and are hidden from view by the extreme for- 
ward extension of the latter. The entire occipital region of Thinohyus siouxensis 1s 
narrow in comparison with that of the recent peccary and the hog. The supraoc- 
cipital continues upward apparently to the top of the occiput, uniting with the 
parietal as in Sus scrofa Linneeus and Phacochewrus xthiopicus Linneus. In the 
peccary the supraoccipital does not extend so high and is met by the downward 
curve of the parietal bones. ‘The parietals in the fossil are irregularly convexo- 
concave. Superiorly, they terminate in the sharp sagittal crest; posteriorly they 
are much extended, to help form the greatly overhanging occiput. The zygomatic 
arch of the squamosal is very robust, especially behind. Below there is a well-formed 
glenoid cayity, similar to that in Platygonus leptorhinus and the peccary, displaying an 
oblique saddle-shaped surface like the glenoid cavity in some carnivora. Supero-pos- 
teriorly the arch terminates in a high, transversely broad, and thin point, with the 
apex gently rounded. At the base of this process (the origin of the posterior temporal 
ridge) on the posterior face of the arch is located the rather small external auditory 
meatus. Postero-laterally, the frontals show sharp and prominent temporal ridges. 
These ridges terminate anteriorly in the rounded and somewhat elevated supercili- 
8 Proc. Amer. Philos. Soc., Vol. XXV., p. 72, 1888. 
