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 
erenulated ; vomerine teeth resembling a single ridge of a dental 
plate. Dorsal and anal fins continuous with the caudal. 
The name Sagenodus was first applied by 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- 
todies in the non-punctate character of the coronal surface. The 
vomerine tooth was originally termed Petcdodopsis by Barkas, on the 
erroneous supposition that it pertained to an Elasmobranch allied to 
Petalodus; and a head with the abdominal region, mingled with 
Palseoniscid 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, R. 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 insequalis, Owen. 
[Plate IV. figs. 2, 3.] 
1867. Sagenodus incequalis, 11. 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 ellipticus, T. Atthey, ibid. p. 87. [Ditto.] 
1870. Ctenodus obliquus, elegans , imbricatus , and ellipticus , Hancock & 
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. Palaeont. p. 28, 
figs. 85, 90. 
1873. Ctenodus elegans, T. P. Barkas, ibid. p. 28, fig, 86. 
1 Geol. Mag. [3] vol. vii. (1890), p. 251. 
