90 FISH-EEMAINS OF THE CAEBONIFEROUS ROCKS. 



Ctenacanthus : but, as Mr. Smith Woodward points out, the 

 fact that the largest spines of Ctenacanthus yet found in 

 Britain occur in our own neighbourhood, and in close connec- 

 tion with the largest discovered teeth of Orodus^ seems to 

 show that the spines of Ctenacanthus and the teeth of 

 Oroclus may have belonged to the same genus of fishes. The 

 spines are most numerous, too, at Armagh, where the teeth 

 of Orodns are found in the greatest abundance. The spines 

 were probably dorsal in position, and would seem to have 

 been deeply imbedded in the back, and therefore immovable. 

 It is quite possible that somewhat similar spines, all classed 

 together as Ctenacanthus^ may have been possessed by several 

 allied genera. The specimens of those species of Ctenacan- 

 thus {C. major^ tenuisfriatus, C. brevis, and C. sidcatus) are 

 in the Bristol Museum. 



Another hook-shaped or strongly arched ichthyodorulite with 

 a broad and shallow base of insertion is described under the 

 generic name oi Phi/sonemus. Small denticles are foimd on 

 the concave edge, and lumpy longitudinal ridges ornament 

 the remainder of the free portion. Two S2:)ecies, Physonemus 

 arcuatus and P. (Onchus) hamatus, have been found in the 

 limestone of this district. The type specimen of hamatus is 

 in the Bristol Museum. 



A broad triangular and somewhat arched ichthyodorulite 

 is known as Oracanthus. It often attains a large size, a 

 specimen in the British Museum (Nat. Hist.) of Oracanthus 

 pustidosus {Phoderacanthus grandls) from Bristol being three 

 feet in length. It is compressed laterally, and the base of 

 insertion is short or altogether absent. The spines are 

 unsymmetrical, and were probably arranged in pairs. Not 

 improbably, as suggested by Dr. Tracjuair, they formed 

 backwardly directed weapons behind the head. It was 

 suo-o-ested by Agassiz that the teeth of Orodns and the 



