/. — Pleuracanthidce. 



705 



Genus Pleuracanthus, Agassiz. '' Recli. sur les Poissons Fossiles," 1837, 



vol. 



111. 



p. 66. 



Diplodus, 

 Orthacanthus, 



Xenacanthus, 



Triodus, 

 Compsacanthus, 



Dittodus, . 



Aganodus, 

 Ochlodus, 

 Pternodus, 

 Trinacodus, 



Lophacanthus, 



Didymodus, 



. Agassiz, L., 1843. " Poiss. Foss.," vol. iii. p. 204. 

 . Agassiz, L., 1843, torn, cit, vol. iii., pp. 177, 330, 



pi. xLv., figs. 7-9. 

 . Beyeich, E., 1848. " Bericht K. Preuss. Akad. 



der Wissenschaften," p. 24. 

 . Jordan, 1849. " Neues Jahrb.," p. 843. 

 . Newberry, J. S., 1856. " Proc. Acad. Nat. Sci. 



Philadelphia," p. 100. 

 . Owen, R., 1867. "Trans. Odont. Soc," vol. v., 



p. 325. 

 . Owen, R., 1867, torn, cit.^ p. 359. 

 . Owen, R., 1867, torn, cit, p. 346. 

 . Owen, R., 1867, torn, cit, p. 363. 

 . St. John and Worthen, 1875. " Palaeont. Illinois," 



vol. vi., p. 289. 

 . Stock, T., 1880. " Ann. Mag. Nat. Hist." series 5, 



vol. v., p. 217. 

 . Cope, E. D., 1883. " Proc. Acad. Nat. Sci. Phila- 

 delphia," p. 108. 



Body comparatively elongated ; skin destitute of shagreen ; head large and 

 depressed ; cranium consisting of segments, more or less ossified, separated by 

 sutures; mouth terminal, widely extended; jaws bearing numerous teeth 

 (Diplodus), arranged as in the Selachii ; notochord persistent ; neural and hsemal 

 arches ossified, the ossification presenting a mosaic appearance due to granular 

 calcifications; seven gill arches (Fritsch) the seventh arch without gills; gill 

 rakers present (Steiiunatodtis) ; head surmounted by a spine, straight or slightly 

 curved ; opening of internal cavity of spine terminal ; along some part of the sur- 

 face extends two rows of denticles ; these may be widely separated and lateral 

 (Pleuracantlms) ; they may be in close proximity along the posterior surface 

 [Orthacanthus) or the two rows may occupy any intermediate position between 

 the two. Attached to the spine is a small cephalic fin ; dorsal fin commencing 

 immediately behind the head extends to the base of the caudal ; the fine rays sup- 

 ported by interspinous and surapophysial bones more numerous than the neural 

 spines ; tail diphycercal with a pointed extremity ; pectoral fins supported by a 

 long articulated axis (archipterygium) with abi-serial arrangement of semi-osseous 



5 L2 



