662 GEOLOGICAL SURVEY OF THE TERRITORIES. 
anterior fontanelle being formed in them in a similar manner, though 
narrower and longer, between the frontals and parietals. The lower 
border of the superoccipital presents a smooth, angular depression, that 
in the articulated vertebra goes to complete the superior third of the 
foramen magnum. 
The lateral bodies of this bone are cellular masses with several aper- 
tures opening forwards and outwards, the mastoids closing them in, in - 
the completed cranium. On its outer surface near the inferior angles 
we observe two, one on either side, grooved foramina, leading upwards 
and inwards, to open into the lateral sinuses on the inner surface of the — 
segment, nearer together. As age advances these canals contract, but 
still exist throughout life. 
‘The basioccipital segment, bo, also is largely cancellous in structure, 
wedge-shaped, having at its apex a long, rounded tubercle curving 
outwards and backwards, overhanging a slight depression beneath it. 
This tubercle in the complete vertebra forms the middle two-thirds of 
the occipital condyle, which, in the adult, is found below the foramen 
magnum, sessile, uniform in outline, with the rounded border below, and 
all indications of its original division into three parts obliterated. 
The neurapophyses of this vertebra, termed the “ exoccipitals” (Plate 
V, Fig. 51, eo), are each nearly as large as the neural spine; on their inner 
borders they present for examination the deeply-rounded margins to 
complete the foramen magnum, and immediately beneath, the minute 
tubercle jutting out that lends its assistance on either side to form the 
condyle of the occiput. 
The outer angles, quadrate in outline, deflected slightly downwards, 
are the transverse processes of the vertebra, the “paroccipitals.” The 
precondyloid foramina are also to be observed here, with one still more 
external, belonging to the group from which the eighth nerve makes its 
way from the cranium. The internal aspect of an exoccipital is a mass 
of open, irregular cells, that are closed in when this segment is approxi- 
mated with the mastoid, superoccipital, and the “‘petrosal”,' (Fig. 51,1), 
that odd-shaped and spongy bonelet which constitutes the capsule of 
the organ of hearing—and which has a foramen on its inner and smooth 
surface for the passage of the auditory nerve—forming, also, by a bend- 
ing forwards of a part of this surface, and aided by the basi-sphenoid, 
the floor of the mesencephalic fossa on either side, while externally it 
shares in forming the entrance from without to the otocrane. 
With the exception of the petrosal, the elements thus far described, 
when duly articulated, form the neural arch of the occipital vertebra, as 
already intimated above. The basioccipital, the centrum of this verte- 
bra, by its larger extremity and the exoccipitals with the connate 
diapophyses articulate with the basi-sphenoid in the basi cranii below; 
the latter, with the superoccipital, meet the parietals and mastoids above 
and laterally. In old birds every trace, both sutural and otherwise, 
becomes completely obliterated as the osseous amalgamation pro- 
gresses, though throughout the group a well-defined “superior line” 
limiting muscular attachment, indicates very nearly the terminating 
borders above, and sometimes, as in Centrocercus, a fainter indication 
exists in the vicinity of the union among the interested bones below. 
On either side of the condyle to its outer aspect we observe in a slight 
depression a group of usually four foramina—two external opening into 
_ 1 Quite recently Dr. Coues has, in an admirable paper, presented us with a review of 
the literature bearing upon the so-called “temporal bone” of human anatomy, and as 
in this paper the true relations and composition of the “‘ petrossal” are so clearly set 
forth, and they express the views now generally accepted, that it gives me pleasure 
