ISCHYODUS. 



189 



Tijpe. — Imperfect palatine dental plate from the Gault of Switzerland. 



Specific Characters. — Mandibular dental plate attaining a length of abont 

 10 cm., very stout, with a deeply sinuous oral margin, acute eminences correspond- 

 ing with the outer tritors, and the beak prominent ; post-oral margin much less 

 vertically inclined than the symphysial margin ; beak-tritor minute ; outer tritors 

 well developed ; median tritor narrow or of moderate width, occupying the greater 

 part of the oral surface immediately behind the anterior outer tritor. Palatine 

 dental jilate with the posterior inner tritor of moderate or large size, and the 

 median tritor not extending so far forwards as this ; outer tritor much elongated. 



Description of Specimens. — The dental plates of this species are most abundant 

 in the Gault, whence the type specimen 

 was obtained. Only two examples appear 

 to be known from the English Lower 

 Chalk. The right mandibular plate, 

 shown of the natural size in Text-fig. 56, 

 A, is more delicate than the typical speci- 

 mens from the Gault and Cambridge 

 Greensand (Text-fig. 56, b, c), with a 

 narrower symphysial facette. It is also 

 peculiar in having the post-oral margin 

 nearly parallel with the anterior margin, 

 and its median tritor is rather remote, 

 being almost entirely behind the anterior 

 outer tritor. A right palatine dental 

 plate from Burham (PL XL, fig. 7) is 

 closely similar to some of the corres- 

 ponding plates from earlier Cretaceous 

 formations referred to this species, but 

 is noteworthy for the deep excavation 

 of the space between the outer and 

 anterior-inner tritors. Its median tritor 

 does not extend so far forwards as in the type specimen from the Swiss Gault. 



Horizons and Lucalities. — Zone of Holasfrr suhglohosus : Tsleham, Cambridge- 

 shire ; Burham, Kent. Also Upper Greensand, Gault, Cambridge Greensand, 

 Pied Chalk, and Lower Greensand. 



Fig. 56. Ischyodus thurmanni, Pictet and Campiche ; 

 three examples of right mandibular dental plate, 

 inner or oral view, nat. size. — Zone of Holaster 

 suhglohosus, Isleham, near Newmarket (a) ; and 

 Cambridge Greensand (b, c). After E. T. Newton. 



2. Ischyodus (?) incisus, Newton. PI. XLI, figs. 5—7. 



1878. Ischyodus incisus, E. T. Newton, Chiraoeroiil Fislies Brit. Cret. Rocks (Mem. Geol. Surv.), 



p. 38, pi. xii, figs. 3—10. 

 1891. Ischyodus (?) incisus, A. S. Woodward, Catal. Foss. Fishes B. M., pt. ii, p. 70. 



