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, Pteraspidse (Fig. 40), the skin-tubercles 



Fig. 40. — Pteraspis rostrata. 

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



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

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

 ind there is often a smaller shield beneath. A separate 

 )iece forms a projecting snout. 



Fig. 41. — Cephalaspis murchisoxi. 

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



(B.M. Guide.) 



The shields of these fishes are often preserved, while the 

 Dmplete body is very rare. 



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



O 



