64 PROFESSOR HIJXLEY ON THE OSTEOLOGY OF THE GENUS GLYPTODON. 
Thus far, no trace of distinct articular processes is visible upon these vertebra ; but 
the hinder face of the eleventh vertebra ( d . 1. 13) presents certain irregular elevations 
and depressions, which interlock with corresponding ridges and cavities of the anterior 
face of the twelfth vertebra. The hinder face of the twelfth (d. 1. 14) and the front 
face of the thirteenth vertebra ( d . 1. 15) are in like case. I shall return to the con- 
sideration of the character of these irregular articular elevations and depressions after 
describing the general form of the vertebrae. 
In all but the first, second, third, eleventh, and thirteenth vertebrae, the parts repre- 
senting the vertebral centra are broken away, but, when they remain, they are so similar 
to one another that their form was, doubtless, essentially the same throughout. Each 
centrum is a comparatively thin bony plate, bent so as to be convex downwards and 
concave upwards, and presenting a much flatter curve in the anterior than in the poste- 
rior part of the column. In front, the central plate is not more than OT inch thick in 
the middle, but it becomes thicker posteriorly, so that the centrum of the eleventh 
vertebra is 045 inch thick; that of the thirteenth vertebra is OT inch thinner. At 
the sides and above, the curved central part of the vertebra passes into the lateral pro- 
cesses and upper arches, which last are slightly concave downwards in the first vertebra, 
flat in the middle vertebrae, and somewhat arched again in the thirteenth. The contour 
of a transverse section of the spinal canal is a transversely elongated oval in the first 
vertebra (fig. 3), is more nearly round, but flattened at the top, in the middle vertebrae 
(d.l. 12), and is a vertically elongated oval in the thirteenth vertebra (d. 1. 15). 
The spinous and transverse processes of the vertebrae are represented by three crests 
or ridges of bone. One of these (Plate VIII. fig. 2, a , b), vertical, and situated in the 
middle line of the dorsal surfaces of the arches of the vertebrae, represents the spinous 
processes; while the lateral crests (<?,<?), directed obliquely upward and downwards, 
answer to transverse, accessory, and mammillary processes. As the latter ridges become 
directed more upwards towards the hinder part of the dorsal region, the total width of 
the column lessens, and the grooves between the middle and the outer ridges become 
deeper in the same direction. Thus, anteriorly, the column is fully six inches broad, 
while at the eleventh vertebra the distance from one external ridge to another is hardly 
half this amount. 
The first vertebra (d. 1. 3) is as broad and depressed as the trivertebral bone. Viewed 
in front (Plate VIII. fig. 3), the neural canal is seen not to take up more than one-fourth 
of the face of the bone, the rest of which is occupied by two broad expanded transverse 
processes, directed very slightly upwards as well as outwards. The under half of each 
of these processes presents an elongated articular facet (a, a'), convex from above 
downwards, slightly concave from side to side, which corresponds with, and is received 
into, the concave articular surfaces upon the hinder face of the trivertebral bone. 
always greater by two than its number reckoned as one of tbe thirteen. In order to avoid confusion in describing 
each vertebra, I shall occasionally give after it its number in the dorsal lumbar series, e. g. (d. 1. 3), (d. 1. 6), 
by which it is indicated in the figures. 
