ARSINOlTIIElllUM. 
5 
The siqrraoccipital {soc.), as already mentioned, is excluded from the foramen 
magnum. It forms the whole of the upper part of the sloping occipital surface, 
which is bounded above by the thickened, rugose, and very prominent lambdoidal 
crest [s.c.]. This seems to be situated along the junction of the supraocci])ital and 
the parietals, though it is impossible to determine ])recisely which is the line of 
union, since even in the youngest specimens the suture is closed. In the middle 
line there is a prominent ridge for the attachment of the ligamentum nuclice., and 
on either side of this the surface is greatly roughened, like the surface of the 
lambdoidal crest. 
The parietals [pa.) are not only fused with the supraoccipital, but also with one 
another in the youngest specimens available for examination. In the immature 
skull figured on PL III. fig. 2 there is, in the middle line of the skull-roof, 
a small foramen, which opens into the sinus occupying this part of the cranial 
wall and is probably a remnant of the sagittal suture. This opening is absent in 
other specimens, and therefore cannot be regarded as of importance. The parietals 
form the anterior portion of the lambdoidal crest, in front of which they constitute 
the skull-roof, which is nearly fiat from side to side, and slightly concave from before 
backwards. Laterally they turn downwards almost at right angles, and help to form 
the side-walls of the skull, their lower edges meeting the squamosals in a nearly 
straight suture, which runs downwards and forwards (PI. III. figs. 1, 2 a). The 
upper edge of this lateral region of the parietals is sharply defined by the strongly- 
marked ridge, which forms the upper limit of the temporal fossa and is continued 
forwards on the frontals as far as the orbits. In the adult the fronto-parietal suture 
is closed, but in the young (PI. III. fig. 2) it can be seen that the parietals extend 
forwards in the middle line between the small frontal horns. Immediately behind 
these the fronto-parietal suture turns forwards to the supratemporal ridge, after 
crossing which it turns downwards and somewhat backwards, running down to 
join the parieto-squamosal suture. 
The squamosal [sq.) is a large and complicated bone which takes a considerable 
share in the formation of the side-wall of the cranium. Its sutures with the 
parietal and exoccipital have already been referred to. It cannot be determined 
whether or not it had any contact with the supraoccipital, but probably its upper 
posterior angle, which helps to form the large lateral occipital prominence, united 
with that bone at least for a short distance. Beneath this prominence the 
surface of the bone is roughened and raised into a ridge, which in the adult 
skull forms a strong, backwardly-directed, hook-like process (text-fig. 1, pr.) for 
the attachment of muscle, situated above and rather behind the opening of the 
external auditory meatus (Pis. I., II. fig. 1, e.a.m). Ventrally the bone is produced 
downwards into a broad post-tympanic flange {])ty.), which terminates below in a 
blunt point and is supported posteriorly by the downward process of the exoccipital 
