THE DEVONIAN PERIOD. 449 



again an evolution of provincial faunas, but on an unusually large and 

 declared scale, and of an advancing phase, verging toward cosmo- 

 politanism. 



The ancestors of the Devonian faunas are found remotely in the 

 cosmopolitan fauna of the Silurian period, and more immediately in 

 the restrictional faunas of the closing Silurian. The early Devonian 

 faunas are to be regarded as the expansional phases of these restrictional 

 faunas. Fortunately the physical conditions during the Devonian 

 evolution are better known than those of previous stages when pro- 

 vincial faunas were undergoing development, and hence reveal in a 

 specially instructive way the conditions under which such faunas 

 develop. Even here, however, the data recorded are far from com- 

 plete, and only a part of them have been critically studied as yet, so 

 that whatever conclusions are now entertained must be held subject 

 to correction. By way of precaution, it is to be recalled that the 

 faunas here under discussion are those of the relatively shallow water 

 that lay about the continental border, or on the bosom of the conti- 

 nent itself, in the form of intruding gulfs and interior seas. The land 

 life, to be considered later, had its own independent phases of develop- 

 ment, while the faunas of the abysmal depths of the ocean and of 

 the surface of the mid-ocean are almost wholly unknown. 



Special faunas evolved. — With the partial withdrawal of the sea 

 from the North American continent, as sketched in the previous 

 chapter, the mid-Silurian fauna underwent restrictive evolution and 

 differentiation in the embayments on the continental border. For 

 want of free communication between these embayments, their faunas 

 developed distinctive aspects. When the sea re-advanced upon the 

 land in the early Devonian, it was by tongues or invading gulfs from 

 different points on the continental border, these being in part exten- 

 sions of the border embayments. Each of these gulfs seems to have 

 had its own rather distinctive fauna, inherited from the embayments 

 of the coast border. 



Five such faunas seem to have invaded the continent of North 

 America during the Early and Middle Devonian, partly in succession 

 and partly simultaneously, viz. : (1) the Helderberg, (2) the Oriskany, 

 (3) the Onondaga (Corniferous), (4) the southern Hamilton, and 

 (5) the northwestern Hamilton. These reached the interior in suc- 

 cession in the order named, and the faunas are described mainly as 



