2 



BRITISH FOSSILS. 



most irregular in shape, some being circular, some elongated longi- 

 tudinally and others transversely. They are coated with a hard 

 enamelloid substance, sculptured with a few radiating sulci. The 

 tubrecles decrease in size on approaching the posterior edge, and 

 become obsolete before they reach it. The hinder rows are more 

 regularly arranged than the anterior ones, forming lines parallel to 

 the back of the spine. On the upper half the tubercles retain the 

 same characters, but are fewer in number and more scattered. They 

 are also intermixed with continuous ridges, similar to those orna- 

 menting the rays of the Hyhodi. Some of them are undulating on 

 the edge, as if they resulted from the confluence of a row of tuber- 

 cles. The angle formed by the junction of the lateral and posterior 

 planes is slightly obtuse on the distal portion, but becomes nearly 

 a right angle at the base. The posterior plane is furnished with a 

 few coarse processes near the point. The root of the spine, and 

 indeed the whole of the surface unoccupied by the superficial orna- 

 ment, is composed of coarse fibrous bone. The line of demarcation 

 between the external and inserted portions of the spine is very 

 oblique, more so than in any other species. These characters, well 

 shown by Mr. Dinkel, in the lithograph representation^ serve to 

 distinguish this from all the members of this genus hitherto de- 

 scribed. 



Affinities. — The irregular arrangement of the tubercles on the 

 sides of this spine is found in Asteracanthus Preiissi (Dunker),* 

 but the latter differs in every other respect. Of the rays described 

 by Professor Agassiz, A steracanthus aciUus is certainly the nearest 

 ally of this species. It is distinguished from it by a more tapering 

 form, the smaller size and more regular disposition of the tubercles, 

 and by the greater number of defensive processes on the posterior 

 surface. The Asteracanthus semisidcatus of Agassiz, has some re- 

 semblance to it in the admixture of ribs with the tubercles, but in 

 other respects it is very distinct. In describing the latter species, 

 Agassiz surmises it to be identical with the Ichthyoclorulites Pur- 

 hecensis of Buckland and De la Beche, but as it is a fossil very 

 characteristic of the Stonesfield oolite, I am inclined to think the 

 Purbeck specimen seen by the latter authors may with more proba- 

 bility be assigned to the species described in this article. In addi- 

 tion to the three new British species of Asteracanthus described in 

 this Decade, I have a specimen of distinct character from the Caen 

 limestone. It is a short, thick spine, densely covered with coarse 



* Palaontographica, vol. i. p. 188. 



