167 



It is, of course, impossible for us to calculate the time 

 occupied by the Silurian era; but from the immense 

 depth of the deposits representing the system, their con- 

 stancy over such vast regions, and the evidence of the 

 organic remains, we may safely conclude that its duration 

 must have far exceeded that of any other; yet each 

 succeeding layer, if only a few inches, tells us of the 

 order and regularity that prevailed, and often leaves us 

 faithful and imperishable remains of the earliest life that 

 existed upon the earth. 



Ascending to the Devonian system, as with each suc- 

 ceedmg advance, we find a continual substitution of 

 genera and species, but never a repetition of one that had 

 previously died out or become extinct. Species seem 

 subject to some such natural law of life and death as 

 individuals. 



In this era the number of Crustacea is much reduced. 

 Fishes are found in so perfect a state of preservation, and 

 in numbers and variety so remarkable, that though they 

 were all very much unlike those of existing nature, we 

 have obtained a very correct knowledge of their struc- 

 ture. It is very interesting to observe, in the upper part 

 of this system, the discovery of the first reptile* (of the 

 Lacertian order), a step higher in the vertebrated series. 

 Of the invertebrata no class has been foimd that is not 

 also in Silurian strata. 



In the Carboniferous system fishes were abundant; 

 the Ganoids t of this era are very interesting, for they 

 appear to have occupied the place of the large reptiles 

 that were destined soon after to replace them. In this 

 system only one true reptile has yet come to light; 

 it is ot the Batrachian order,J so that we may conclude 

 they were but rare; and the sauroid character of the 



• Tclerputon Elgiiicnse. f MegaliclUhys, Sic. 



X l*aral)atracliiis Colci. 



