﻿256 SIREN01DEI. 



Body depressed, covered with large thin scales, almost quadrate 

 in shape, but having the angles well rounded; both scales and 

 external bones destitute of a ganoine-layer. A large median 

 occipital plate posteriorly, with a smaller median plate immediately 

 adjoining the front margin of this element. Dental plates, above 

 and below, triangular, irregularly ovate or elliptical in form, with 

 few strong, outwardly directed ridges, more or less tuberculated or 

 crenulated ; vomerine teeth resembling a single ridge of a dental 

 plate. Dorsal and anal fins continuous with the caudal. 



The name Sagenodus was first applied bj- Owen to a horizontal 

 microscopical section of a dental plate ; while that of Ptyonodus was 

 given by Cope to dental plates differing only from those of Cera- 

 todus in the non-punctate character of the coronal surface. The 

 vomerine tooth was originally termed Petalodopsis by Barkas, on the 

 erroneous supposition that it pertained to an Elasmobranch allied to 

 Petalodus ; and a head with the abdominal region, mingled with 

 Palaeoniscid scales, formed the type of Megapleuron, Gaudry. On 

 account of the limited extent to which the ridges of the dental 

 plates are tuberculated in the adult, the type species was associated 

 by Jaekel, evidently in error, with a Ceratodont species from the 

 Muschelkalk, and re-named Hemictenodus ; and, without any allu- 

 sion to synonymy, B. H. Traquair recently 1 adopted the latter 

 term, while pointing out the essential feature in the diagnosis, i. e. 

 the disposition of the median occipital bones. 



Sagenodus inaequalis, Owen. 

 [Plate IV. figs. 2, 3.] 



18G7. Sagenodus incequalis, B. Owen, Trans. Odontol. Soc. vol. v. 



p. 365, pi. xii. 

 1868. Ctenodus obliquus, T. Atthey, Ann. Mag. Nat. Hist. [4] vol. i. 



p. 84. [Dental plate ; Newcastle-upon-Tyne Museum.] 

 1868. Ctenodus elegans, T. Atthey, ibid. p. 86. [Ditto.] 

 1868. Ctenodus imbricatus, T. Atthey, ibid. p. 86. [Ditto.] 

 1868. Ctenodus elltpticus, T. Atthey, ibid, p. 87. [Ditto.] 

 1870. Ctenodus obliquus, elegans, imbricatus, and ellipticm, Hancock St 



Atthey, Nat. Hist. Trans. Northumb. & Durham, vol. iii. pp. 63- 



66. 



1872. Ctenodus obliquus, elegans, and' imbricatus, Hancock & Atthey, 

 ibid. vol. iv. p. 407, pi. xiii. figs, 1-3. 



1873. Ctenodus obliquus, T. P. Barkas, Coal Meas. Paleeont. p. 28, 

 figs. 85, 90. 



1873. Ctenodus elegans, T. P. Barkas, ibid. p. 28, fig. 86. 



Geol. Mag. [3] vol. vii. (1890), p. 251. 



