xi THE GEOLOGICAL RECORD 193 



forms, the whole surface being covered with small quad- 

 rangular flattened tubercles. The tail is somewhat twisted 

 to show the bi-forked character. The mouth must have 



FIG. 39. THELODUS SCOTICUS. 

 From Upper Silurian, Lanarkshire. Half nat. size. (B.M. Guide.) 



been an aperture underneath the head. Good specimens of 

 these are rarely preserved. 



In another family, Pteraspidae (Fig. 40), the skin-tubercles 



FIG. 40. PTERASPIS ROSTRATA. 

 From Old Red Sandstone of Herefordshire. One-third nat. size. (B.M. Guide.) 



are united into plates and scales, while the head is covered 

 with a dorsal shield, often terminating behind in a spine ; 

 and there is often a smaller shield beneath. A separate 

 piece forms a projecting snout. 



FIG. 41. CEPHALASPIS MURCHISONI. 



From Old Red Sandstone of Herefordshire. About half nat. size. 

 (B.M. Guide.) 



The shields of these fishes are often preserved, while the 

 complete body is very rare. 



Another group (Fig. 41) has the head shield continuous 



O 



