THE OSTEOLOGY OF HY^NODON. 503 
^ The skell „f is thoro.ghly creedoet i„ character, as may be seen 
dhIi c™7v|"”"’T’h in^pacious cranium, the short distance betwee/the occi- 
p.ta condyles and the postgleno.d process, the high s,,gitlal crest, the deep post- 
orb.tal constrichon. and the relatively short preorbital region of the face The 
upper profile of the skul hes in almost the same plane from the nose to the occiput, 
but tins appearance is largely due to the sagittal crest, which becomes very high 
at Its junction with the lambdoidal crest, the roof of the cranial cavity inclining 
Steeply downward and backward from the postorbital constriction, and reaching a 
very low level at the occiput. 
The basioccipital is short, but very broad and thin, concave on the dorsal sur- 
face and slightly concave on the ventral, which has a short, feebly marked keel in 
the median line. The other occipital elements early coalesce into a single mass, so 
that even in young specimens it is by no means easy to determine their limits. As 
a whole, the occiput is usually very high (it is lower in H. paucide7is) and of sub- 
triangular shape, with broad bcase and lanceolate apex. The width of the basal 
l)ortion is largely due to the transverse expansion of the exoccipitals, which form 
a convexity in the median line to receive the vermis of the cerebellum, and on each 
side of this is a shallow depression. The paroccipital processes are short, narrow, 
and antero-posteriorly compressed and flattened. The condyles are rather small, 
low, Jind depressed, but quite strongly divergent from each other; the foramen 
magnum is small and subcircular in shape. The supraoccipital is high and almost 
pointed at the summit, though its shape varies in the difierent species; its upper 
portion is diploetic, filled with cancellate tissue, and develops a bony tentorium. 
The exocci])itals are separated from the lambdoidal crest by a broad surface of the 
peri<jtics, terminating distally in small rugose mastoid processes, which, like the 
paroccij)ital processes, stand but little in advance of the condyles. The tympanic 
is loosely attached to the skull, and is very generally missing from the specimens; 
when present, it is a small and moderately infiated bulla. The external auditory 
meatus is iniperi’ectly ossified, and forms but a partial tube. 
The basisplmwid is quite long, broad at the suture with the basioccipital but 
narrowing forward. The presphenoid is narrow and but little of it is displayed, 
even when the palatal tube is broken away, since it is largely concealed by fiie 
vomer. The alisphenoid is quite large, but forms only a limited part of the side 
wall of the cranium ; near its anterior edge is an oblique overhanging ridge, winch 
runs forward and upward, and is continued upon the frontal, until it passes into the 
iio-storbital process, d'he pterygoid process of the alisphenoid is large and ap- 
proaches near to its fellow of the opposite side, being in most of the species sepa- 
rated only bv the narrow cleft of the posterior iiares. In H. kptocephalus there is 
a sutural union between the two processes, concealing the pterygoids and causing 
the posterior iiares to open backward rather than downward The limits of he 
orbit!, splienoids are not clearly visible in any of the specimens, but it is obvious that 
these bones must have been small. 
The pamtak are very long and form almost the entn-e roof of the ceiebral 
