610 GEOLOGICAL SURVEY OF THE TERRITORIES. 
the bases of these concavities, points opposite the posterior openings of 
the ilio-neural canals, being the narrowest part of the pelvis. The upper 
and at the same time the inner margins of the bones in question, from 
the anterior and median angle, at first approach, soon to diverge from 
each other, and form the gluteal ridges and borders of those scale-like pro- 
jections of the posterior portion of the ilia that overhang the acetabula. 
Produced now as the “gluteal ridges,” they tend almost directly back- 
wards, though very slightly inwards, to terminate in the ischial mar- 
gins. The preacetabular dorsal iliac surfaces are generally concave, 
while the postacetabular, and at the same time that surface which 
occupies the higher plane, is flat, having a slope downwards and 
backwards, with a ventral reduplication after forming the rounded and 
concave posterior boundary of the pelvis. The preacetabular super- 
ficial iliac area is nearly double the extent of the postacetabular. 
The antitrochanterian facets that surmount the cotyloid cavities have 
the usual backward direction, though their surfaces look downwards, 
outwards, and alittle forwards. The external surfaces of the ischia look 
upwards and outwards, having just the reverse direction ventrally. 
Posteriorly, these bones are produced beyond the ilia into finely pointed 
extremities, tending to approach each other. The slender pubic bones, 
after closing in the obdurator foramen on either side, touch and unite 
with the inferior borders of the ischia as far as the pointed ends of the 
latter, beyond which they are produced nearly to meet behind. ‘The in- 
terval between the free extremities of the pubic bones in some individ- 
uals, notably “birds of the year,” is very slight, less than a millimetre 
sometimes, approaching a closed pelvis. The circular and thoroughly 
perforated acetabula are formed in the usual manner by the three pelvic 
bones. They have a diameter of about three millimetres, and their cir- 
cumferences are in the vertical plane. The ischiadic foramina are ellip- 
tical and large; they are, as usual, posterior to the acetabula and above 
the obdurator foramina. These last are also elliptical, and about one- 
third the size of the others. Should the major axes of these two ellipses 
be produced backwards, they would intersect and form an acute angle 
just within the posterior pelvic border. Viewing the pelvis ventral- 
wise, we observe, in addition to points mentioned when speaking of the 
sacrum, the reduplication of the ilia, forming pockets behind and inter- 
nally, that open outwards through the ischiadic foramina and inwards 
into the general pelvic cavity. The pelvic passage is subcircular, un- 
closed, with an average diameter of 1.7 centimetres vertically, and a 
little less transversely. The narrowest part of the pelvis measures 1.2 
centimetres, the widest 2 centimetres, being taken between the iliac pro- 
jections over the acetabula; the average length, including anterior neural 
spine, is 3 centimetres. Pneumatic foramina occur in the shallow an- 
tfractuosities, between the antitrochanters and gluteal ridges in the ilia. 
None of the caudal vertebrw are grasped by the pelvis, the posterior ex- 
tremity of the sacrum always assisting to form the curve of the pelvic 
passage. The usual number of these vertebree is seven, though occa- 
sionally an additional one is found, making eight in some individuals. 
This enumeration does not include the modified and ultimate coccygeal 
vertebra, the pygostyle. They are all freely movable upon one another, 
and the first upon the last sacral vertebra. The articular facets upon 
the centra vary in shape throughout the series; that upon the first is 
long transversely, with a double convexity so arranged as to accommo- 
date itself to the one on the extremity of the sacrum; they soon become 
uniform, to pass to the subcircular one existing between the last verte- 
bra and the pygostyle, on which it is concave. 
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