148 



ORDERS OF FISHES. 



In the genus Pterasjns (fig. 513) the head is defended, as 

 in Cephalaspis, by a shield or buckler, which is composed of 

 several pieces firmly anchylosed. The shield consists of a 

 central disc, the lateral angles of which are produced into 

 short cornua, wliilst it is extended into a 

 rostrum in front. The posterior spine is 

 very small, and is attached to the disc as 

 a separate piece. The orbits are situated 

 laterally. The minute structure of the 

 shield is very complex, consisting of three 

 layers. The innermost layer is laminated, 

 and is traversed by vascular canals. The 

 middle layer is made up of polygonal 

 cavities ; and the outer layer is struc- 

 tureless or fibrous, and is finely striated 

 or grooved. The body was covered with 

 scales; but nothing is known of the nature 

 of the fins. The genera Cyathas-pis and Scapliaspis have been 

 founded upon forms which have usually been placed under 

 Fteraspis, and which differ in more or less essential points 

 from the typical species of this genus. The genus Pteraspis, 



Fig. 5113. — Cepliali 

 buckler of rteraspls (Cy- 

 ntlMspis) BanksH. From 

 the Upiier Ludlow rocks 

 of Ludlow. (After Mur- 

 I'liison.) 



Fig. 514.— Ceiihalic shield of Coccosteus decipiens, viewed on one side, as restored by Pander 

 —Old Red Sandstone. The surface-ornamentation is omitted, but a small portion is repre- 

 sented at a, on a larger scale. 



SO far as yet known, comprises the most ancient of the fishes, 

 commencing as it does in the earlier portion of the Ludlow 

 formation (Upper Silurian). Other species are known in the 



