624 BRITISH FOSSIL REPTILES. 



Species — Chondrosteosaurus magnm. (' Dinosauria,' Plates S3 — 85.) 



In the subject of Plate 84 sufficient of the concave articular surface is preserved 

 to show its correspondence in size with that of the subject of Plate 80, but its pro- 

 portions are reversed, the vertical diameter plainly appearing to surpass the trans- 

 verse one. The present vertebra, it is true, has come from a more posterior part 

 of the column. The parapophysis has disappeared, at least from the position from 

 which it projects in the subject of Plate 79 : if such process was present its origin 

 has risen to near the base of the neural arch. So much of the free surface of the 

 centrum as remains is concave lengthwise ; all trace of flattening of the inferior 

 surface has disappeared. The curve of the free surface toward the fore end of the 

 centrum indicates that vertebral element to have been shorter absolutely, and much 

 more so relatively to the hinder cup, than in Cliondrosteosaurm (/igas. It is hard 

 to suppose that so extreme a degree of modification of shape and proportion should be 

 present in an anterior and a middle dorsal vertebra of the same spine or in the same 

 species, as is exemplified by the subjects of Plates 80 and 84 ; I therefore refer them 

 to distinct species. The present vertebra agrees more closely in proportions with that 

 of which a side view is given in Plate 83. 



The centrum is shorter in proportion to both breadth and height than in Chondro- 

 steosaurus gigas. The rise in the position of the parapophysis shows the vertebra (Plate 

 83) to have come from a more posterior part of the spinal column than the subjects of 

 Plate 79, and of fig. 1, Plate 82. The outlet of the side-pit is shorter and deeper 

 (vertically) ; yet the long diameter of the aperture is about one third that of the centrum ; 

 its compact lining layer of bone is entire. The fore end of the centrum shows the con- 

 vexity, the hind end the concavity, characteristic, with the chondrosal texture of the bone 

 (Plate 85), of the present remarkable genus. The neurapophysial bases extend to within 

 an inch and a half of the hind margin of the centrum ; they rise at the beginning of the 

 convexity of the fore end. This convexity has suffered abrasion, and the widely cancellous 

 structure is exposed, as shown in Plate 85. 



It seems not needless to remark, in reference to such fossils, that the primal basis of 

 the vertebrate skeleton may be converted into sclerine or chondriue, and that ossification 

 may begin in either ' membrane ' or ' cartilage.' In some vertebrates, chiefly if not 

 exclusively cold-blooded, more or less of the bone may remain unossified, retaining the 

 antecedent stage, with some slight modification of tissue, to which, as in selachian 

 vertebrae, the term ' chondrine ' has been applied. Such partially ossified bones, when 

 petrified, show corresponding cavities, usually filled with matrix or spar. 



But this condition of fossil bones may depend on other osteogenetic changes. After 

 substitution of bone-earth for gristle, or the conversion of the entire cartilaginous mould 



