20G 
TI<:KTIARY VEJITEBRATA OF THE FAYtJM. 
by the a])])arontly loosely united periotic {per.). The relations of the bones in this 
region seem to be very similar to those found in Manatus, except that in the latter 
genus the squamosal is not entirely cut off from contact with the supraoccipital. 
lleneath the ])criotic opening the post-tympanic portion of the squamosal unites 
closely with the anterior face of the paroccipital region of the exoccipital, and in front 
forms the posterior border of the external auditory meatus, the anterior boundary 
of which is constituted by the slightly developed postglenoid process. The articular 
surface {gl.) for the mandible is very broad ; it is slightly concave from side to side 
and gently convex from before backwards, except where it runs on to the slight post- 
glenoid ridge. The zygomatic process {~gg.) is very stout and is triangular in section ; 
it is underlain anteriorly by a slender prolongation of the jugal. The gmrietals {pa.) 
are large bones forming most of the cranial roof ; posteriorly, as already mentioned, 
they are separated in the middle line by a process of the supraoccipital, and laterally 
they send back processes which thrust themselves between the squamosal and 
supraocci})ital, and help to form the large prominences of the lambdoidal ridge. From 
the posterior angles of the bones a pair of not very well-defined supratemporal ridges 
run forwards, first converging slightly, then running parallel to about the level of 
the anterior border of the glenoid surface; in front of this they diverge considerably, 
and finally become continuous with the hinder edges of the postorbital processes 
of the frontals. 
Owing to the comparatively small development of the supratemporal ridges and 
also to the slight inflation of the cranial portion of the skull, the roof is not nearly 
so sharply marked off from the temporal fossiu as in the later forms, even in Eosiren. 
The ventral borders of the parietals unite with the squamosals posteriorly, then for 
a short distance with the upper end of the alisphenoid («/.), and in front of this again 
with the frontals, which they overlap in a squamous suture, running forwards in 
a sharp angle on tlie supratemporal ridges. The frontals (fr.) are very wide, forming 
the broad gently convex skull-roof in the region of the orbits, which they overhang 
as large supraorbital plates. Posteriorly they join the alisphenoid and in front 
of this their ventral edges seem to form the outer border of the groove for the optic 
nerve. Their anterior border is notched for the reception of the hinder ends of the 
nasals, between which they send a short triangular process. The nasals (w«.) seem to 
have been of considerable size, and overhung the narial o])ening {nar.) to some extent. 
The oj)ening itself is an elongated oval in outline, and its borders are formed by the 
nasals behind and by the ])rcmaxillae in front and at the sides. The ])remaxillary 
rostrum must have been bent down to a considerable extent, but in this s])ecimcn it 
has been straiglitened out by the pressure to which it has been subjected, and tlic end 
has been broken across tlie alveoli of the anterior incisors, 'flic u])])cr surface of the 
rostrum in front of the narial o])ening is gently concave from side to side. 
The maxilla {mx.) is a greatly elongated bone ; the anterior edge of its palatine region 
