274 PETKIFACTIONS AND THEIR TEACHINGS. CHAP. III. 



relative proportions of the apophyses ; the figure is the size 

 of the original. 



Fig. 4 of the same lignograph is a lateral view of a caudal 

 vertebra, remarkable for the deep cavity left between the 

 centrum and the base of the neurapophysis, (o) by the re- 

 moval of the pleural element or transverse process, which 

 had dropped out before the bone was imbedded in the 

 sandstone. 



In the caudal vertebrae the centrum is more cuneiform than 

 in the dorsals, the sides are smooth and almost flat vertically, 

 and but slightly depressed in their antero-posterior diameter ; 

 at their inferior convergence, instead of uniting in a rounded 

 ridge as in the dorsals, they are separated by a deep longitu- 

 dinal furrow, bounded anteriorly and posteriorly by the 

 oblique extremity of the centrum, which is truncated at both 

 ends to articulate with the inferior spinous process or chevron- 

 bone (see Jig. 3, Lign. 35). The articular faces of the bodies 

 correspond with those of the posterior dorsals ; the anterior 

 surface is almost flat, the posterior slightly concave ; the 

 neural arch no longer presents the peculiar characters ob- 

 servable in the dorsal and lumbar vertebrae ; it is attached 

 by a wide base to the body, and the two laminae in some 

 instances extend transversely over the latter so as to complete 

 the spinal canal, as in many of the dorsal vertebrae. The 

 anterior zygapophyses (which are shown in Lign. 57) have 

 their elliptical articular surfaces almost vertical, and closely 

 embrace the corresponding posterior processes; the latter 

 spring off from the base of the neural spine, and project over 

 the centrum. The neural spine, or spinous process, is very 

 long ; it rises by an anterior basal ridge from the neural arch 

 as in the dorsals, but is greatly contracted at its commence- 

 ment, and increasing in breadth as it ascends, terminates 

 in a thick truncated summit. The longest spine in the 

 specimen before us is nearly 16 inches in height, and 2 

 inches in antero-posterior diameter at the summit ; the spine 

 is thin in a transverse direction, the truncated summit is 

 but \ inch thick. The transverse processes are relatively 

 short and strong. The height from the base of the centrum 

 to the top of the spinous process is 22 inches, and as the 

 chevron-bone, when perfect, would be nearly five inches in 

 length, the vertical expansion of the tail in the young Igua- 



