554 



FISHES. 



as;pis: head covered by a continuous shield with tubercular 



surface, produced into a 

 horn at each posterior 

 corner; a median dorsal 

 backward prolongation 

 bears a spine; hetero- 

 cercal. Auchenaspis and 

 Didymaspis: allied to the 

 preceding, but with the 

 cephalic shield divided 

 into a larger anterior and 

 smaller posterior piece. 

 Fteraspis: with the ceph- 

 alic shield finely striated 

 or grooved, composed of 

 seven pieces. Scaphaspis 

 and Gyathaspis: with the 

 surface of the head-shield 

 similarly sculptured as in 

 Pteraspis, but simple in 

 the former, and composed 

 of four pieces in the latter. 

 Astrolepis: attained to the 

 gigantic ^ size of between 

 twenty and thirty feet; 

 its mouth was furnished 

 with two rows of teeth, 

 of which the outer ones 

 were small, the inner 

 much larger. 



[See Ray Lankester, A. 

 Monograph of the Fishes 

 of the Old Red Sandstone 

 of Britain. Part I. Ceph- 

 alaspidffi. Lond. 1868 

 and 1870. 4to.] 



Q 7^ 



a 



