ICHTHYODORULITES, 
135 
Genus OR ACANTHUS, Agassiz. 
[Poiss. Poss. vol. iii. 1837, p. 13.] 
Syn. Platyacanthus, F. M‘Coy, Ann. Mag. Nat. Hist. [2] vol. ii. 1848, 
p. 120. 
Pm ye acanthus, St. John & Worthen, Pal. Illinois, vol. vi. 1875, 
p. 480. 
Phoderacanthus, J. W. Davis, Trans. Hoy. Dublin Soc. [2] vol. i. 
1883, p. 533. 
Spines attaining to a very large size, much laterally-compressed, 
usually unsymmetrical, broad and triangular, rarely elongated and 
slightly arched ; internal cavity very large, base of insertion short 
or absent. Sides of exserted portion ornamented by large tubercles, 
with a tendency to arrangement in transverse series, sometimes 
fused. 
As observed by J. W. Davis 1 , the broad triangular spines of this 
genus are unsymmetrical and must have been arranged in pairs; 
the lower margin of one side of each spine being straight, while the 
internal cavity on the other side is exposed by a great excavation. 
Such spines have subsequently been discovered by R. H. Traquair 2 , 
forming a pair of backwardly-directed weapons behind the head of 
an Elasmobranch; and microscopical sections have proved the 
absence of boue-corpuscles in their structure. The spines were 
originally supposed by Agassiz 3 to be referable to Orodus ; argu¬ 
ments in favour of their pertaining to Psammodus were afterwards 
discussed by R. Etheridge, Jun. 4 5 ; and more recently they have been 
regarded by Inostranzeff J as not improbably the spines of Poly- 
rhizodus. 
The narrow elongated spines seem to be homologous with the 
typical spines named Gyracanthus, while the broader examples cor¬ 
respond to the thin, hollow, triangular bodies also met with in the 
last-named genus. 
Oracantlras milleri, Agassiz. 
[Plate I. fig. 3.] 
1837. Oracanthus milleri, L. Agassiz, Poiss. Foss. vol. iii. p. 13, pi. iii. 
figs. 1-4. 
1 Trans. Roy. Dublin Soc. [2] vol. i. p. 530, pi. lxiv. fig. 1. 
2 Geol. Mag. [3] vol. v. (1888), p. 86. 
3 Poiss. Foss. vol. iii. p. 171. 
4 Geol. Mag. [2] vol. iv. (1877), p. 308. 
5 Trudui St. Peterb. Obsheh. Estest-Ispuit. vol. xiv. (1888), p 16-. 
