378 PROCEEDINGS OF THE CANADIAN INSTITUTE. 
of the notochord does not take place between the fourth and third 
vertebree. Nor does it do so between the second and third, because 
there is only one notochordal plug between the comparatively flat 
posterior face of the first vertebra and the deep conical hollow of the 
anterior face of the conjoined second and third vertebre. 
Dorsally the first vertebral centrum is quite free from that which 
follows, but ventrally they are suturally united by delicate plates 
which dovetail into each other on either side of the aortic canal. Also 
the anterior face of the first centrum is comparatively flat, much more 
so than the posterior face of the basioccipital bone against which it 
abuts. The dorsal surface—that which looks into the neural canal— 
has two sockets, separated by a narrow median partition, (Fig. 3, Pl. 
VI.) In these, rotate freely, the permanently cartilaginous balls 
which represent the proximal parts of the first neural arches, and 
which, in fact, are the articular processes of the ‘stapedes.’ It will be 
observed from Fig. 12, Pl. [V., that more cartilage is present in the 
first vertebra than in any of those which succeed it. Fig. 8a represents 
the form of the complete stapes. Besides the ‘articular’ it possesses 
two other processes, which are merely ossified in membrane ; these 
are the slender, ‘ ascending’ process which les in the neural canal 
immediately in front cf the point of emergence of the second spinal 
nerve-roots, and the spoon-shaped ‘anterior’ process which does not 
form part of the wall of the neural canal, being separated from the 
spinal cord by a diverticulum of dura mater, the atriwm sinus im- 
paris, (Figs. 12 and 13, Pl. IV., and 4, Pl. VI.), to the lateral wall 
of which the spoon-shaped process fits closely. 
It is obvious that the anterior process of the stapes passes beyond 
the anterior face of the vertebra to which it belongs. It rests upon 
the exoccipital at the side of the posterior aperture of the cavum 
sinus imparis, immediately below the foramen magnum (Fig. 5, Pl. 
VI.) and its rounded anterior border fits into a notch on the posterior 
margin of that bone, which is very distinct in a profile view of this 
part of the skull. 
Returning to the madleus it will be remembered that its tip also 
projects in front of the body of the first vertebra. The internal edge 
of the tip wil] be found to be connected by a stout ligament whose 
fibres have a tendinous lustre with the roughened lateral surface of 
the spoon-shaped process of the stapes. In the ligament is a small 
bone—the incus—irregularly oblong in the adult, but style-shaped in 
