400 



GEOLOGICAL SUCCESSION OF ANIMALS. 



kind. Even those fishes which have a more regular shape, 

 as the Dipterus, have not horny scales like our common fishes, 



Fig. ZlS.PtericJithys, from the Devonian rocks of Scotland. Agass. 



but are protected by a coat of bony plates, covered with 

 enamel, like the gar pikes (Lepidosteus) of the American 

 rivers. Moreover they all exhibit certain characteristic fea- 

 tures, which are very interesting in a physiological point of 

 view. They all have a broad head, and a tail terminating in 

 two unequal lobes. What is still more curious, the best 

 preserved specimens show no indications of the bodies of the 

 vertebrae, but merely the spinous processes ; from which it must 

 be inferred that the body of the vertebra was cartilaginous, as 

 it is in our sturgeons. 



668. Recurring to what has been stated on that point in 

 Chapter Twelfth, we thence conclude that these ancient fishes 

 were not so fully developed as most of our fishes, being, like 



