﻿1861.] 



HUXLEY PTEKASP1S DTTNEN3IS. 



165 



called my attention to the specimen, without giving me any informa- 

 tion as to its previous history, I at once affirmed it to be a Pteraspis, 

 — being led to this determination by the eminently characteristic 

 striation of the outer surface, combined with the no less peculiar 

 polygonal cells of the middle layer*. There is nothing like either 

 of these tissues in any Cephalopod or Crustacean with which I am ac- 

 quainted — the construction of the cuttle-bone being totally different ; 

 and they exist, in combination, in no animal structure which has yet 

 been described, except Pteraspis. In form, and in the presence of the 

 diverging ridges described by Prof. Roemer, the fossil perfectly agrees 

 with many of our English Pteraspides ; and I have therefore no 

 hesitation in expressing the opinion that Archceoteuihis must disappear 



Diagram of a restored Pteraspis. 

 A B 



a. Snout or rostrum, united with b, the shield-like disk. c. The cornua of the 

 latter ; d. its median backward prolongation ; e. the median posterior spine into 

 which the last is produced. /. Orbits or nasal apertures. 



* I have carefully described these structures in my memoir " On Cephalaspis 

 and Pteraspis," Quarterly Journal of the Geological Society, 1858, vol. xiv. 

 VOL. XVII. PART I. N 



