H. G. SEELEY ON THE AXIS OF A WEALDEN DINOSAUR. 463 



median basal line (fig. 1). The under surface of the centrum is ex- 

 panded anteriorly, being widened from side to side by the parapo- 

 physes (figs. 1 and 2, A), which are large tubercles terminating in flat, 

 vertical, ovate, lateral surfaces, which look backward and outward ; 

 each measures -| inch high and -| inch wide, and is placed below the 

 middle of the side of the centrum, and about \ inch behind the ante- 

 rior articular surface ; the transverse width of the vertebra through 

 the parapophyses is nearly 3 inches. Behind the parapophyses the 

 centrum has a constricted, aspect measuring 1-| inch from side to 

 side at | inch from the posterior end; in this line are two small 

 foramina about an inch apart. The under surface has a slight 

 median ridge, which does not extend forward to the wedge-bone ; 

 on each side of this the bone is flattened and a little convex from 

 below upward, except under the parapophyses, where it is concave. 

 A channel | inch wide separates the parapophysis from the diajjo- 

 physis. Above this channel is a prominent strong tubercle pro- 

 jecting transversely from the side of the neural arch. This second 

 articulation for the rib (g) is flat, nearly circular, with a slightly 

 elevated border; it looks outward, downward, and very slightly 

 backward ; and more than half its diameter lies behind the parapo- 

 physis, which it slightly overhangs. 



The side of the neural arch is transversely oblong (fig. 1), concave 

 in length and concave from above downward. In front, above the 

 neural canal, it is compressed from side to side so as to form a pro- 

 minent broad longitudinal ridge (i) rather than a neural spine. It is 

 -J inch wide in front, narrower in the middle, and widens again 

 behind ; it is slightly convex in length, nearly horizontal, and less 

 than 2 inches long, 



On the middle of the side of the neural arch, on the same level 

 with the top of the diapophysis, but in front of it and separated 

 from it by a concave notch, is the small, thin, diverging, anterior 

 zygapophysis (figs. 1 and 2,f), which is directed forward, outward, 

 and downward. The anterior zygapophyses measure about 2 inches 

 from side to side ; and below them the bone is slightly compressed 

 (fig. 2). The distance from the anterior to the posterior zygapophyses 

 measures 2-f- inches, while the space between the anterior and poste- 

 rior vertebral notches for the intervertebral nerves measures 1^ inch 

 in length. 



The posterior zygapophysis is large and projects backward (fig. 1, 1), 

 is convex from side to side, and looks downward and a little outward ; 

 it projects nearly \ inch behind the neural arch, and measures -J inch 

 in length and in breadth. The neural arch is excavated above and 

 between the posterior zygapophyses ; and the neural canal appears 

 to be larger behind than in front. 



This vertebra presents considerable resemblance in front to the 

 axis of a bird, in the whole form and character of the articular sur- 

 faces of the zygapophyses, centrum, and dentata, in having a hypa- 

 pophysial wedge-bone (although it is much smaller than in birds), 

 and in the absence of a neural spine. Moreover, in those young 

 birds in which the cervical rib is not yet blended with the vertebra 

 Q. J. G. S. No. 123. 2 i 



