64 flllPHPmiH CHIMiEROIDS. 



the Plagiostomous order, among fishes, to include so many genera 

 which exhibit in the dental substance the rich organization of vas- 

 cular and calcigerous tubes that has already been described, and the 

 modifications of which become the more important and interesting, 

 when, as in the case of the extinct Psammodus, Acrodus, and Hybodus, 

 they are almost the sole features of the organization of those most 

 ancient vertebrate animals which remain for the contemplation of 

 the anatomist and physiologist. 



CHIM^ROIDS. 



25. I next proceed to consider a well marked modification of the 

 same highly organized dental structure in the genus ChimcBra, which, 

 like the Cestracion, is an extreme but still more anomalous modifica- 

 tion of the chondropterygious type, having the branchiae unattached 

 externally : this genus is also the representative of several extinct forms 

 of fishes. 



The jaws of the Chimseroid fishes are armed, says Cuvier, in the 

 characters assigned to this family in the Regne Animal, (1) with hard 

 and indivisible plates instead of teeth, four above and two below, and 

 in the 2nd edition of the Lecons d'Anatomie Comparee,(2) these 

 dental plates are described as being salient, trenchant and striated. 

 They have not, as yet, been more particularly described, but the 

 coarse medullary tubes which enter into their composition, and to 

 which Cuvier alludes when he compares the structure of these teeth 

 with those of the Orycteropus in his Histoire Naturelle des Pois- 

 sons,(3) are illustrated in PL 40, fig. 20, of the great work on Fossil 

 Fishes by M. Agassiz. 



The teeth of the Callorhynchus or ChimcBra Australis, are 

 represented as they appear on looking into the widely opened 

 mouth in Plate 28, fig. 1. The upper jaw presents two anterior 

 dental plates, of a small size and semi-elliptic form, and two posterior 



(1) Tom. ii, p. 382. 



(2) Tom. iv, p. 362. 



(3) Tom. i, p. 496. Cuvier's words are, " Leur tissu interieur est perce de tubes fins, 

 comme un jonc ou comme les dents de I'orycterope." The tubes here mentioned are the medul- 

 lary or vascular canals, and are merely the centres from which the true calcigerous tubes 

 radiate. — See Report of British Association, 1838, p. 145. 



