﻿WEALDEN. 



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would be nearly sixteen inches in length if it came from the spinal column of a Bothrio- 

 spondylus with an anterior dorsal or cervical vertebra of the vertical and transverse 

 dimensions of the subject of PI. VIII. 



I infer, therefore, that this fossil indicates a species with proportions of the vertebra?, 

 and probably of the trunk, more like those deducible from the type of Bothriospondylus 

 robustus. But the present remarkable fossil shows a still larger proportion of unossified 

 parts in the substance of the centrum. 



The side-pit is short, vertically wide, but deep ; the long diameter of the aperture is 

 somewhat more than one third that of the entire centrum : its compact lining layer of 

 bone is entire, not perforated as in the 'foramina pneumatica' of birds. The fore part of 

 this element is strongly convex ; the hind part answerably concave. The bases of the 

 neural arch 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 abra- 

 sion, and the widely cancellous structure is exposed, as shown in PI. IX. 



Craving excuse for premising so trite or elementary remarks, the primal basis of the 

 vertebrate skeleton may be converted into sclerine or chondrine, and ossification may 

 begin in either ' membrane ' or 1 cartilage.' In some Vertebrates, chiefly if not ex- 

 clusively cold-blooded, more or less of the bone may remaing unossified, retaining the 

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

 the term ' chondrine ' has been specially given. 



Such partially ossified bones show corresponding cavities, usually filled with matrix, or 

 spar, in the petrified state. 



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 

 into bone, the central part may be absorbed, and an oily substance called ' marrow ' be 

 deposited in the cavity. Or, the absorption of previous solid bone may go further, the 

 marrow may also be removed, the wall of the bone may be perforated or 'tapped,' and 

 air be admitted from a contiguous extension of the lung. 



But in cases of petrifaction the non-ossified parts of a bone become filled by the same 

 mineral infiltration, whether the cavities originally contained chondrine, marrow, or air. 



The inconsiderate conclusion that fossil bones with large cavities and thin compact 

 walls must have been those of Pterodactyles or of Birds led to the supposition that 

 certain fossil eggs belonged to one or other of these volant classes, the bones of the 

 unexcluded embryo showing the above hollow or tubular character. Such eggs in a 

 portion of stone from a quarry in the Island of Ascension were submitted to my examina- 

 tion in 1834, and I detected among them the characteristic scapula and coracoid of a 

 Chelonian embryo. To the objection against that determination, based on the hollowness 

 of the larger limb-bones associated therewith, I showed, by dissection of a newly hatched 

 Chelone preserved in spirits in the Hunterian Museum, that the cavity of such bones had 

 been filled with chondrine, not with air; that the thin outer shell of bone was a transitory 



