592 GEOLOGY 



and showed in their teeth and skull-bones, many resemblances to 

 the amphibians, of which they were perhaps the ancestors. Like 

 the lung-fishes, they appear to have been at or near the climax of 

 their evolution at this time, though they lived on in large numbers 

 to the Cretaceous. Sharks (elasmobranchs) are now chiefly marine, 

 and (unless the Paleozoic era be excepted) have been so throughout 

 known geological time, though a few live in fresh water, as in Lakes 

 Nicaragua and Baikal. In the Devonian period they seem to have 

 lived in the open sea, but their remains are also found in the Old 

 Red Sadstone and equivalent formations, so that they probably 

 lived in fresh and brackish waters, as well as in the ocean. 



Shells believed to have belonged to fresh-water mollusks, and 

 closely resembling living genera, have been found, in association 

 with land plants and fishes. Little is known of the fresh-water 

 vegetation. 



III. The Land Life 



The known land life of the Devonian period consisted of plants, 

 snails, insects, myriapods, scorpions, and traces of amphibians; 

 but the record of land life is very imperfect. In the early stages 

 of its evolution, vegetation was doubtless more perishable than 

 later; but even now plant tissues are less well fitted for fossilization 

 than the shells and skeletons of animals. The normal fate of up- 

 land plants is to perish where they grow, and to disappear by decay, 

 consumption, combustion, or some other form of destruction. 

 The chances of the prompt burial and preservation of lowland 

 vegetation are better; but it is only when the low land is being 

 rapidly aggraded, that the conditions for the fossilization of its 

 vegetation are even fairly favorable. Even when preserved, it is 

 rare that leaves, fruit, twigs, limbs, trunk, and roots are all pre- 

 served together. So true is this that in the case of ancient plants. 

 especially if not closely analogous to modern types, it is more or 

 less hazardous to attempt to restore the original plant by com- 

 bining the dissevered parts of different individuals. 



The Devonian period covers much of the early development, 

 though probably not the actual beginning of terrestrial plant life. 

 It saw the origin of ferns, scouring rushes, lycopods, the seed- 



