70 PICTURESQUE SKETCHES 



several groups characteristic of the period we are 

 now considering. But another group also, containing 

 four genera, is worthy of notice, as contrasting 

 strongly with the Cephalaspides (as the former are 

 called); and, instead of being clothed with large 

 plates, these are recognised by the extremely mi- 

 nute scales with which the fish belonging to it are 

 covered. These scales give to the skin an appear- 

 ance very strikingly resembling that of shagreen. 



The size of the fishes thus brought together is ge- 

 nerally small, and their shape is squat and awkward, 

 the head being large, and the body dwindling away 

 to a very small tail : they have, however, large teeth, 

 and must have been powerful, if not very rapid fishes. 

 Their fins offer some peculiarities, being formed of a 

 multitude of delicate-jointed rays, generally termi- 

 nated by one very powerful ray or spine, sometimes 

 simply planted in the flesh, sometimes articulated to 

 bone. Both this group and the former are entirely 

 confined to the first epoch, and almost entirely to the 

 particular period of the old red sandstone. 



Another group of these ancient fishes (Dipterians*) 

 is remarkable for the great magnitude to which the 

 fins were developed, and the fact that in all of 

 them the fins on the back and below the tail are 

 double. The jaws of these animals were provided 

 with sharp-pointed teeth ; the head was inclosed as 

 if in a box of cartilage, coated with enamel ; and the 

 scales of the body are in some species so large as not 

 to have required more than about half a dozen to 

 reach from head to tail (Glyptolepis). This, however, 



* Aig (<Zzs), twice ; Trrtpov (joferow), a fin : double-finned. 



