188 FOSSIL FISHES OF THE ENGLISH CHALK. 



slender, laterally compressed, and ordy slightly curved, with the flattened sides 

 ornamented by irregular longitudinal strife (PI. XLI, figs. 3, 4). The anterior 

 border is compressed to a sharp edge, which is surmounted by a slight, beaded 

 ridge of enamel, while the grooved posterior face (fig. 4) bears a close series of 

 downwardly-curved, enamelled denticles (fig. 4 a) along each edge for the greater 

 part of its length. The spine is hollow and is cleft behind at its base, where it 

 originally united with the supporting cartilage. 



Genus ISCHYODUS, Egerton. 



Ischyodus, P. M. G. Egerton, Proc. Geol. Soc, vol. iv, 1843, p. 155. 



Generic Characters. — Mandibular dental plate usually massive, with a well- 

 defined hard layer on the outer aspect immediately below the oral margin ; its 

 symphysial facette narrow ; one anterior tritor usually present, sometimes several ; 

 one median tritor, and two or more outer tritors. Palatine plate also massive, 

 with outer thickening ; four tritors present, two being inner, one median, and one 

 outer. Vomerine plate more or less quadrate in side view, with tritors on the oral 

 margin ; post-oral region not laterally expanded, and usually with a definite hard 

 thickening. Dorsal fin-spine laterally compressed, more or less longitudinally 

 ridged, sometimes even rugose, with a double series of posterior denticles. Head- 

 spine of male short, arched, with a terminal cluster of denticles. 



Type Species. — Ischyodus toimiseudi (Agassiz, Poiss. Foss., vol. iii, 1843, p. 343, 

 pi. xl, figs. 20 — 22, pi. xlr, figs. 17, 18), from the Portlandian of S. England. 



Remarks. — This genus is typically of Jurassic age, and is known by nearly 

 complete skeletons from the Lithographic Stone of S. Germany. Only rare 

 fraorments occur in the Chalk. 



"O' 



1. Ischyodus thurmanni, Pictet and Campiche. Plate XL, fig. 7; Text-figure 56. 



1843. Ischyodus brevirostris, P. M. Gr. Egerton, Proc. Geol. Soc, vol. iv, p. 156 (name only). 

 1843. Chimpera (Ischyodon) brevirosh-is, L. Agassiz, Poiss. Foss., vol. iii, p. 344 (name only). 

 1843. Ohimpera (Ischyodon) agassizii, L. Agassiz (errore), torn, cit., pi. xl c, figs. 14, 15. 

 1858. Ischyodus thurmanni, Pictet and Campiche, Foss. Terr. Cret. Ste Croix (Pal. Suisse), p. 76, 



pi. ix, fig. 8. 

 1862. Ischyodus agassizii, W. H. Bensted (errore). Geologist, vol. v, p. 378. 

 1867. Ischyodus bouchardi, H. E. Sauvage, Catal. Poiss. Second. Bouloimais (Mem. Soc. Acad. 



Boulogne-sur-Mer, vol. ii), p. 81, pi. iv, fig. 6. 

 1876. Ischyodus brevirostris, E. T. Newton, Quart. Journ. Geol. Soc, vol. xxxii, p. 326, pi. xxi, fig. 5. 

 1878. Ischyodus brevirostris, E. T. Newton, ChimEeroid Fishes Brit. Cret. Eocks (Mem. Geol. Surv.), 



p. 27, pi. ix. 

 1891. Ischyodus thurmanni, A. S. Woodward, Catal. Foss. Fishes B. M., pt. ii, p. 67. 



