CHIMyEROIDS. 



67 



stance of the jaw, which presents the coarse cellular structure of the 

 bone of osseous fishes. The teeth are not, however, loosely implanted 

 in these alveolar cavities, but wherever they are in contact with the 

 bone, they are anchylosed thereto. Their structure is, nevertheless, 

 strikingly different from that of the jaw itself, and corresponds with 

 the modification of the tubular structure presented by the teeth of the 

 Chimtsra. In the Edaphodon there are three of these large teeth on 

 each side of both jaws : their position is nearly horizontal and their 

 grinding surface is pitted with small impressions.(l) 



In the tooth of the Chimsera, the complex structure arising, as it 

 were, out of an aggregation of long slender and simple cylindrical teeth 

 is most conspicuous. Each of the parallel medullary or vascular 

 canals, which from their large size are easily distinguishable by the 

 naked eye, is the centre of radiation to very numerous and closely 

 compacted calcigerous tubes, having a diameter of 77^3-^ th of an inch at 

 their origin, but ramifying and diminishing in size as they recede, 

 and ultimately terminating in a minute irregular cellular ossification, 

 which occupies the interspaces of the different systems of radiated 

 tubes, and cements them together in a continuous mass in the recent 

 dental plate. The course of the calcigerous tubes is at right angles to 

 the medullary canal, and they have a wavy disposition ; their primary 

 branches are sent off at an acute angle, and the fine ramuli from these 

 branches dilate or terminate in minute cells considerably smaller than 

 the radiated cells of mammiferous bone. 



In the fossil teeth of the large extinct Chim^rcs, the coarser cel- 

 lular structure, which cements together the systems of calcigerous 

 tubes, is generally more or less decomposed, while the denser parietes 

 of the medullary canals formed by the calcigerous tubes remain ; the 

 coarse tubular structure of the tooth thus displayed, is accurately 

 figured by M. Agassiz in the Chimcera Townsendii. But the medullary 

 canals and their radiated systems of calcigerous tubes are not only 

 cemented together by the ossified capsules, but are still more effectually 

 connected together by the frequent anastomoses of the parallel medul- 

 lary canals themselves. The portion of the tooth figured (PL 29, fig. 2) 

 exhibits some of these anastomoses. In those parts of the section in 



(1) Proceedings of the Geological Society, 1838, p, 687- 



f2 



