HABITAT OF THE EARLY VERTEBRATES 411 



apparition, it must be observed, was due to the exceptional 

 preservation of the land record. Thereafter, by interpretation, the 

 habitats of both the Agnatha and the Pisces were more general 

 and varied, a portion taking permanently to the sea, and a por- 

 tion remaining in the land waters, while a third portion migrated 

 between the two. The well-known habit of many of the last to 

 return to the swift inland streams to initiate each new genera- 

 tion is suggestive of ancestral conditions. The sharks, the hag- 

 fishes and many Teleostomes represent divisions that became 

 permanently sea faring. The lung-fishes (Dipnoans), the old- 

 type Crossopterygians, a part of the lampreys, and many 

 others seem to have mainly adhered to the fresh water, at least 

 their present representatives now frequent these waters, either 

 exclusively or in the main. The fossil record of this latter group, 

 throughout the later geologic ages, is well nigh as scant as that 

 previous to the Devonian, and this would seem to be a very sig- 

 nificant fact. The lampreys seem to have been ancestrally rep- 

 resented in the Devonian by the little Palmospondylus gunni 

 recently found in beautiful preservation in the Achanarras 

 quarry in North Scotland, where conditions for the preservation 

 of fresh-water deposits were exceptionally good. After this 

 single appearance the lamprey type was lost to sight until mod- 

 ern times revealed its probable descendants in the lamprey of 

 our present waters. In like manner, the Dipnoans, after a nota- 

 ble record in the Devonian and Carboniferous periods, where 

 fresh-water life was exceptionally preserved, nearly disappeared 

 from the record, but are now found in three forms, one in 

 each of the three southern continents, Africa, Australia, and 

 South America. So, too, the singular Crossopterygians, though 

 their deployment may have been wider, after a fine display in 

 the Devonian and Carboniferous, passed into a decline in the 

 early Mesozoic and disappeared in the Lias, but are now found 

 in the fresh waters of Africa. Special interest attaches to the 

 Dipnoans and Crossopterygians as the probable ancestors, or 

 the nearest known kin to the ancestors of the amphibians, and 

 through them, of all reptiles, birds, and mammals, because it 



