320 [Dec. 18, 



one long continuous tunnel or arched tubular bridge of bone, a struc- 

 ture which is without a parallel among the Mammalian Vertebrata. 

 Of these thirteen vertebrae, the four [anterior are so completely an- 

 chylosed together, that the original lines of demarcation between 

 them are hardly discernible. Persistent sutures separate the fourth 

 from the fifth, and the latter from the sixth ; but all trace of the 

 primitive distinction of the sixth and seventh is lost. The other 

 vertebrae are separated by sutures which become coarser and less close 

 posteriorly. In all but the first, second, third, eleventh, and thir- 

 teenth vertebrae, the parts representing the vertebral centra are broken 

 away ; but where they persist, they are so similar that they were 

 doubtless of similar form throughout. Each centrum is, in fact, a 

 comparatively thin bony plate, so curved as to form a segment of a 

 hollow cylinder of much larger diameter in the front than in the 

 hinder vertebrae, the sides of which pass superiorly into the arches of 

 the vertebrae. 



The foremost vertebra of the thirteen is as broad as the posterior 

 part of the 'trivertebral bone,' and presents a couple of convex arti- 

 cular facets which articulate with the lateral articular concavities 

 described above in that bone. The vertebrae rapidly narrow, how- 

 ever, until the fourth is not more than three-fifths as wide as the 

 first, while it is proportionately deeper ; and this increase of depth 

 relatively to width goes on until in the thirteenth vertebra the spinal 

 canal is deeper than it is wide. 



The spinous processes of these vertebrae are all broken short off ; 

 but sufficient remains of their bases to make the following points 

 clear. 



The spinous process of the first is almost obsolete, being a mere 

 ridge sloping back towards the second, with which it is continuous. 

 This appears to have been necessary to afford the requisite play for 

 the knob of the trivertebral bone in its movements of flexion and ex- 

 tension on the rest of the spinal column. 



The spinous process of the second vertebra was long and thick, 

 and probably somewhat high. It appears to have been completely 

 distinct from the third, which was thinner, and was anchylosed with 

 its successors (as far as that of the twelfth vertebra inclusive) into a 

 long continuous crest. The apices of the spinous processes may, 

 however, have been distinct. So much as is left of the base of this 



