EOSIREN LIBYCA. 
201 
border of the narial opening. This aperture is an elongated oval in outline, its long 
diameter being about twice its width. In front of it the upper surface of the rostrum 
is broad and rounded from side to side, but towards its extremity it becomes compressed 
and its upper border forms a sharj) angle. At the end of the premaxillae there is a pair 
of large though comparatively shallow sockets for the enlarged incisors (?. 1), separated 
by a thin w'all of bone. Behind and beneath these the alveolar border forms a sharp 
edge continuous with that of the maxillae. The palatal surface of the premaxillae is 
strongly arched and posteriorly forms the roof of the large anterior palatine foramen 
(PI. XX. fig. 1 A, a.p.f.), the lower border of which, as already mentioned, is formed 
by the anterior edge of the maxillae. Posteriorly the palatine foramen opens into the 
narial cavity; its form and relations differ somewhat from those of the same opening 
in Manatns, Ilalicore, and lihytina. In Manatus the palatal portions of the premaxillae 
are scarcely at all developed ; in Ilalicore they are in the same plane as, and unite 
Avith, the palatine plates of the maxillae, the lateral borders of the elongated anterior 
palatine foramen being formed equally by the two bones. In Rhytina the elongated 
foramen is almost entirely enclosed by the premaxillae ; in the present genus it 
approximates much more nearly to the condition seen in llalitherium, in which, 
however, the palatine region of the premaxillae is rather less developed. 
The comer seems to be much like that of llalitherium, as figured by Lepsius ; it is 
deeply grooved and the nasal opening must have been divided, at least posteriorly, by 
a vertical cartilaginous septum. 
In all the specimens the zygomatic arch is broken and incomplete. Judging, 
however, from an imperfect specimen, it appears that the structure Avas much as in 
llalitherium, the zygomatic process of the squamosal forming a broad blade, the loAver 
edge of Avhich is underlain by the backAvard process of the jugal. This latter 
widens out in its middle region and bears a doAvuAvardly-directed blunt process, as 
in llalitherium. The body of the squamosal forms a considerable portion of the side 
Avail of the cranium. Posteriorly it unites Avith the supraoccipital above and perhaps 
for a short distance beloAv this Avith the exoccipital ; but for the most part it is separated 
from the latter bone by the mastoid portion of the periotic, Avhich is exposed on the 
occipital surface as an elongated convex mass. In front of its union Avith the mastoid 
the squamosal bears a strong rounded ridge, forming the outer portion of the lambdoidal 
crest. In front of this there is a concave surface bounded in front by the ridge marking 
the origin of the zygomatic process. 
The base of the skull is for the most part Avanting ; in the type specimen, hoAvever, 
it can be seen that the basisphenoid and presphenoid Avere both much thickened and 
that the former is embraced by the upper ends of the massive pterygoids. 
The cast of the brain-cavity (text-fig. G5) is of considerable interest. The olfactory 
lobes [ol.) are small and situated Ioav doAvn. The hemispheres are divided from one 
another by a very deep groove in front, and each is divided into a swollen anterior 
2 D 
