﻿ICHTHYODORULITES. 



135 



Genus ORACANTHUS, Agassiz. 

 [Poiss. Foss. vol. iii. 1837, p. 18.] 

 Syn. Platyacanthus, F. M'Coy, Ann. Mag. Nat. Hist. [2] vol. ii. 1848, 

 p. 120. 

 Pnigeacanthus, St. John & Worthen, Pal. Illinois, vol. vi. 1875, 



p. 480. 

 Phoder acanthus, J. W. Davis, Trans. Roy. 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 shorb 

 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 E. H. Traquair 2 , 

 forming a pair of backwardly-directed weapons behind the head of 

 an Elasmobranch ; and microscopical sections have proved the 

 absence of bone-corpuscles in their structure. The spines were 

 originally supposed by Agassiz 3 to be referable to Oroclus ; argu- 

 ments in favour of their pertaining to Psammodus were afterwards 

 discussed by R. Etheridge, Jim. 4 ; and more recently they have been 

 regarded by InostranzefT 5 as not improbably the spines of Poly- 

 rhizodus. 



The narrow elongated spines seem to be homologous with the 

 typical spines named Gyracanihus, while the broader examples cor- 

 respond to the thin, hollow, triangular bodies also met with in the 

 last-named genus. 



Oracanthus milleri, Agassiz, 



[Plate I. fig. 3.] 



1837. Oracanthus milleri, L. Agassiz, Poiss. Foss. voL iii. p. 13, pi. iii. 

 figs. 1-4. 



1 Trans. Eoy. 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. 



* Geol. Mag. [2] vol. iv. (1877), p. 308. 



5 Trudui St. Peterb. Obshch. Estest-Ispuit. vol. xix. (1888), p 16. 



