VERTEBRAL COLUMN 



283 



the centra or bodies of the vertebrae are formed by the pairs 

 of iuterventralia, while the basiventralia are reduced, persisting 



Fig. 56. — Composition of vertebrae of Reptiles, illustrated by the first and second 

 cervical vertebrae. (1) Atlas (first cervical) and axis (second) vertebra of Croco- 

 .dilus. (2) Atlas and axis of Metriorhynchus, a Jurassic Crocodile. (3) Analysis 

 of the first two cervical vertebrae of a Crocodile ; 2, second basiventral complex 

 or " intercentrum " continued upwards into the meniscus or intervertebral pad. 

 (i) Diagram of the fundamental composition of a Reptilian vertebra ; compare this 

 and (6) with Fig. 1 (8 and 9) on p. 13. (5) The first three cervical vertebrae of 

 Sphenodon. (6) Trunk-vertebrae oi Eryops, a Permian Proreptile ; typically temno- 

 spoudylous ; cp, articular facet of the capitulum of a rib. (7) Tlie complete atlas 

 of an adult Trionyx hurum ; the second basiventral (intercentrum) is attached to the 

 posterior end of the first centrum, which, not being fused with the second centrum, 

 is not yet an odontoid process. (8) The complete atlas of an adult Trionyx gange- 

 iicus ; still typically temnospondylous. (9) The first and second cervical vertebrae 

 of an adult Platemys. (10) The complete atlas of a Chelys fimhriata. Az, Anterior 

 zygapophysis ; B.D, basidorsal ; B. V, basiventral ; Cj, Oi, C3, first, second, and third 

 centra, formed by the interventralia ; Op^, Op% articular facets of the capitular 

 portions of the first and second ribs ; /. V, interventral ; JYi, A\, iVj, first, second, 

 and third neural arch, formed by basidorsalia (B.D) ; Od, odontoid process = first 

 centrum ; Fz, posterior zygapophysis ; M^, R^, ribs ; Sp, detached spinous process 

 of the first neural arch ; t^, t^, tubercular attachments of the first and second ribs ; 

 1, 2, 3, 4, " intercentra " = basiventrals ; 7, //, ///, position of the exit of the first, 

 second, and third spinal nerves. 



either as so-called intercentra or wedge -bones, or as inter- 

 vertebral pads, or disappearing altogether. At the earlier 

 stages of development the gastrocentrous vertebrae behave in the 



