2 
ICHTHYOTOMI. 
Genus PLEURACANTHUS, Agassiz. 
[Poiss. Fosb. vol. iii. 1837, p. 66.] 
Syn. Diplodus, L. Agassiz, Poiss. Foss. vol. iii. 181.% p. 204. 
Oflhacanthus!, L. Agassiz, tom. cit. 1843, pi. 4.5, figs. 7-9. 
Xenacanthiis, E. Beyricli, Monatsb. Verb. k. Prenss. Akad. Wiss. 
1848, p. 24. 
Triodus, Jordan, Neues Jahrb. 1849, p. 843. 
Compsaeanthtm, J. 8. Newberry, Proc. Acad. Nat. Sci. Pbilad. 
1856, p. 100. 
Dittodus, It. Owen, Trans. Odont. See. toI. v. 1867, p. 326. 
Ayanodtm, R. Owen, tom, cit. p. 3.59. 
Ochlodtis, R. Owen, tom. cit. p. 346. 
Pternodm, R. Owen, tom. cit. p. .363. 
Thrinacodu^, St. John & Worthen, Pal. Illinois, vol. vi. 1876, 
p. 289. 
Lophacanthus, T. Stock, Ann. Mag. Nat. Hist. [6] vol. v. 1880, 
p. 217. 
Anodontacanthus, J. W. Davis, Quart. Joum. Geol. Soc. 
vol. xxxvii. 1881, p. 427. 
Didymodu. 1 , E. D. Cope, Proc. Acad. Nat. Sci. Pbilad. 1883, 
p. 108. 
Body probably destitute of shagreen ; a long barbed spine, with 
two series of denticles, placed dorsally immediately behind the head. 
Tooth with thick depressed root, the crown consisting of two prin- 
cipal divergent cones, generally of unequal size, with a more or less 
minute intermediate denticle, and often a round flat-topped boss 
(“ button ”) behind. 
The generic identity of the fishes bearing the spines named 
Plmracanthus and Xenacanthns, and the teeth named Diplodus, was 
first pointed out by Egerton ’. Triodus was also referred to the 
same genus by Schnur and Orthacanthus by J. W. Davis *. Zittel 
ha.s already remarked" upon the probable identity of CompsacantJius 
with the so-called Orthacanthus ■, and Traquair has determined® the 
identity of Anodontacanthus, in part, with the spines of the present 
genus. The name Thrinacodus was given by St. John and \Yorthen to 
teeth differing from those Diplodus'’ in the absence of a “ button ” 
a character apparently inconstant, as noted by Cope Dittodus, 
‘ Ann. Mag. Nat. Hist. [2] vol. xx. (1857), p. 423. 
® Zeitschr. deutsch. geol. Gesell. vol. viii. (I860), p. 542. 
® Quart. .Toum. Geol. Soc. vol. xxxvi. (1880), p. 331. 
" Handb. Palseont. vol. iii. pt. i. (1887), p. 90. 
® Geol. Mag. [3] vol. v. (1888), p. 101. ’ 
