210 GEOLOGY. 



imals characteristics of the reptiles which are to abound 

 so largely in one of the following ages as to give it its 

 name. When such mingling of the characteristics of two 

 or more classes of animals occurs in one animal, it is said 

 to be a comprehensive type. Such types were quite com- 

 mon in the forming ages of the world, but as it advanced 

 to its fully-developed condition, in preparation for the ad- 

 vent of man, the classes of animals became more distinct- 

 ly defined, and at the present time such comprehensive 

 types are rare exceptions so rare as to be regarded as 

 great curiosities in nature. One of these is the duck- 

 billed Platypus of Australia, noticed in 133 in my Nat- 

 ural History. 



302. Tails of the Ancient Fishes Nearly all the fishes 



whose remains are found 

 in the earlier strata of the 

 earth's rocks had unilobed 

 tails, as seen in Fig. 120 

 that is, the spinal column 

 Fi s- 12 - ran into the upper lobe of 



the tail, the lower being so small 



as not to be counted as a lobe. 



In the later strata, on the oth- 

 er hand, the tails are bilobed, 



Fig. 121, in the great majority 



of the fossils. At the present 



time the only family of fishes * rig. 121. 



that has the unilobed tails is the Shark family. 



303. Abundance of Devonian Fishes. In estimating 

 the abundance of ancient fishes from their fossils, we are 

 to consider how uncommon a thing it must be for fishes 

 to become enveloped in sand or mud, in comparison, for 

 example, with shells. Keeping this in view, the great 

 number of species that have been made out from the fos- 

 sils in some quarters, especially in Great Britain and Eu- 

 rope, indicates that fishes must have been very abundant 

 in the Devonian age. In some cases great numbers must, 



