208 



THE WOELD OF LIFE 



Passing through the long series of Lower Silurian strata, 

 (now separated as Ordovician) we have fuller developments 

 and more varied forms of the same classes found in the Cam- 

 brian; but in the Upper Silurian we meet with remains of 

 fishes, the first of the great series of the vertebrates to appear 

 upon the earth. Thej are of strange forms and low type, 

 mostly covered with a kind of plate-armour, and apparentlv 

 without any lower jaw. Hence they form a separate class — 

 Agnatha (^^ without jaws"). They also appear to have had 

 no hard, bony skeleton, as the only parts fossilised are the outer 

 skin with its more or less armoured covering. The illustra- 

 tion (Fig. 39) shows one of the simpler forms, the whole sur- 



FiG. 39. — Thelodus scoticiis. 

 From Upper Silurian, Lanarkshire. Half nat. size. 



(B.M. Guide.) 



face being covered with small quadrangular flattened tubercles. 

 The tail is somewhat twisted to show the bi-forked character. 

 The mouth must have been an aperture underneath the head. 

 Good specimens of these are rarely preserv^ed. 



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.) 



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

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



