26 Prof. Owen on an Ichthyolite from Sheppey. 



m, fig. B, an appearance affecting at first sight a general charac- 

 ter attributed to the bones of fishes *, and apparently at variance 

 with the kno^vn laws of development of the osseous tissue in the 

 existing members of the class. 



In most Vertebrata, as is well known to physiologists, exten- 

 sion of parts is not the sole process which takes place in the 

 growth of bone : to adapt the bone to its destined office, changes 

 are wrought in it by the absorption of parts previously formed, 

 chiefly in the interior. In the growth of the bones of fishes 

 such internal changes have not been observed, and hence the 

 character assigned to them by Prof. M. Edwards ; and in point 

 of fact, most of the bones of recent fishes are solid or spongy in 

 their interior. The bones of the Chelonia are likewise solid : a 

 coarse diploe fills the interior of the long bones of the extremi- 

 ties, and we find a similar structure in the bones of the Cetacea 

 and Phocidce. Among terrestrial mammals also, the inactive 

 Sloths, both recent f and extinct {, have the long bones of the 

 extremities solid ; whilst the agile Antelopes have their diaphyses 

 in the condition of hollow columns; the strength and lightness 

 of the bones being increased by the progressive absorption of the 

 ' first-formed osseous substance, which is removed from within as 

 new bone is deposited from without. The ribs of the large 

 Ophidians, which serve them as legs, have likewise their medul- 

 lary cavities ; and the bodies of the vertebrse of some Lizards, 

 and of the great extinct Poikilopleuronj are similarly excavated. 

 These medullary cavities become filled with spar or matrix in 

 fossil marrow-bones : and the same infiltration of foreign matter 

 in the cavities of such bones of cartilaginous fishes, as the jaw 

 of the Myliobates here described, might seem to indicate that 

 there had been an original formation of a medullary cavity in it 

 by the action of the absorbents on a primitively solid bone. 

 This however is not the case : in most Chondropterygii an os- 

 seous crust is formed upon the periphery of the original cartilage ; 

 the crust consists, as in the fossil (figs. A, B, c), of prismatic 

 pieces, which under the microscope present oval calcigerous cells 

 about 3^^o^h of an inch in diameter, but without conspicuous 

 radiating tubes : the ossicles closely resemble in tissue the plates 

 and tubercles (placoid scales) on the integument ; but in the 

 fossil this tessellated crust of bone may be traced passing beneath 

 the posterior dental plates. 



The cavities which such partially ossified bones of fishes ap- 

 pear, when seen in the fossil state, to have had while recent, were 



* " Les OS lie presentent jamais de canal medullaire," Milne Edwards, 

 Elemens de Zoologie, Classe des Poissons, p. 690, 1834. 

 f De Biainville, Ost^ographie de Paresseux, p. 1. 

 + Owen, Memoir on the Mylodon, 4to, pp. 83, 112. 



