EllITISH FOSSILS. 



Decade YIIL Plate I. 



ASTERACANTHUS GRANULOSUS. 



[Genus ASTERACANTHUS. Agassiz. (Sub-kmgdom Vertebrata. Class Pisces. 

 Order Placoidei. Family Cestraciontidse.) Dorsal spine large, tuberculate, with a double 

 row of processes on the posterior margin ; base smooth.] 



Aster acanthus granulosus, Sp. Nov. 



Description. — The cliaracters assigned by Professor Agassiz to the 

 genus Aster acanthus, are striking, constant, and unmistakeable. 

 The leading features of these Ichthyodorulites are the tubercular 

 surface of the dorsal ray, and the stellate ornament of the tubercles. 

 In the typical species, A steracanthus ornatissumis, found in the 

 Kimmeridge clay, these characteristics attain their maximum de- 

 velopm.ent — in the species under consideration, from the Tilgace beds, 

 they are reduced to the minimum. The specimen represented (tig. 1. 

 of the Plate) is the only one approaching a perfect state I have met 

 with. It probably belonged to a young individual. The other 

 fignre is taken from a fragment of a much larger ray, in the British 

 Museum, belongirg to the same species. The length of the former 

 is nearly I foot; when perfect it probably measured an inch more. 

 The base at the front of the ray is only 4 inches in length, but the 

 cavity at the back extends upwards of 8 inches. It is scarcely pos- 

 sible that the whole of this can have been imbedded in the muscles ; 

 it is more likely that it supported a large adipose or membranous 

 fin, attached to the hinder surface, and embracing the cavity, but 

 leaving the rough outer part of the bone exposed for offensive or 

 defensive operations. The whole texture of the bone is remarkably 

 coarse and fibrous. It is traversed by inosculating canals, inter- 

 spersed with pores, arranged in longitudinal series, and showing a 

 reticulated pattern, when examined with a lens. The root and the 

 hinder surface as high as the termination of the cavity are smooth, 

 and free from ornament. The remainder of the ray is covered by 

 numerous sm_ooth tubercles, isolated^ but arranged in longitudinal 

 [vili. i.] 8 B 



