794 THE WONDERS OF GEOLOGY. Lect. VIII. 



those which now impart the perception of light to the living 

 Crustacea. The mutual relations of light to the eye, and of 

 the eye to light, were, therefore, the same at the time when 

 crustaceans first existed in the bottom of the Silurian seas, 

 as at the present moment."* 



23. Fishes of the Silurian System. — Of vertebrated 

 animals, the relics of a few species of fishes belonging to five 

 or six genera of the placoid order {ante, p. 340), are the only 

 vestiges hitherto obtained from the immense series of strata 

 composing the Silurian system. Even imprints of the feet 

 of a higher order of animals, Reptiles, of which the carbon- 

 iferous rocks afforded indications, are altogether wanting in 

 these most ancient fossiliferous deposits. Until lately, 

 Ichtbyolites were only known in the upper Silurian ; but I 

 learn from Sir R. Murchison, that the same species of 

 Cestracion (Onchus Murchisoni) that occurs in the upper, 

 has recently been found in the lower deposits : the same 

 general organic type, therefore, prevails throughout the 

 system. 



These ichthyolites consist of teeth, portions of the skin or 

 shagreen, and dorsal rays or spines, of fishes of small size.f 

 They have chiefly been found in the upper Ludlow rocks, 

 in which there is a bone-bed, a few inches thick, almost 

 wholly made up of animal detritus, consisting of the scales, 

 teeth, fin-bones, coprolites, &c. of small fishes: this deposit 

 resembles the bone-bed at the base of the Lias in Somerset- 

 shire (ante, p. 529). In the Wenlock shale, numerous 

 teeth and fin-bones of fishes of the Cestracion family have 

 been discovered. 



24. The Cumbrian or Schistose System. — The 

 Silurian system is succeeded by a vast series of strata of a 

 slaty character, which are destitute of any distinct assem- 

 blage of organic remains, although fossils occur in some of 

 the uppermost rocks. These deposits extend over a great 



* Bridgewater Essay. f Bee PL IV. of Silurian System. 



