THE PALEONTOLOGIC RECORD 341 



I can not in this place analyze the peculiarities which give the 

 austral fauna of these " Falklandia " strands their special impress but 

 I may specially cite the trilobites which are astonishingly developed. 

 I presume any competent student of northern faunas, being shown a 

 series of these without knowledge of their origin, would pronounce 

 them of early Devonian age and yet they are neither northern species 

 nor, in any large degree, northern genera. While they bear the im- 

 press of boreal genera and resort to morphologic equivalencies thereto 

 in fugitive epidermal structures which so richly characterize the boreal 

 trilobites at this time, they are on the whole constructed on a series of 

 modified types which hold their fundamental expression while de- 

 veloping minor details with the chronology normal to their succession 

 at the north. The Phacopes are seldom true Phacopcs, the Dal ma- 

 nit es seldom true Dalmanites, yet the same structural decorations and 

 extravagances we are familiar with at the north, are distributed freely 

 through the group. This is all equally true, in qualifying terms, of 

 the other groups of this fauna, save for the fact that in these we can 

 hardly venture to insist so entirely on generic distinctions south and 

 north. The species differences declare themselves on every hand and 

 taken as a whole the fauna presents fairly conclusive evidence of hav- 

 ing derived its distinctiveness through its isolation from the boreal 

 fauna from which it ancestrally took origin. Yet while it has de- 

 veloped this character it has also proceeded to maintain a faunal com- 

 position which declares its age, and a morphological stamp which 

 shows that it developed all its parts in the proper time and place in the 

 series. 



In predicating geographic isolation as the prime factor in this 

 regional development of the Devonian fauna, its efficiency should not 

 be made to seem qualified by an illustration which is striking by 

 virtue of its contrast with the already well known. There are evi- 

 dences in plenty that geographic isolation has played a similar role 

 with even more diverse effect in the development of the boreal faunas 

 of the same geologic stage. The north Atlantic land bridge was con- 

 tinuous at this time, as evidenced not alone by the presence of the 

 Coblentzian fauna in the Atlantic coast rocks but by an array of addi- 

 tional facts ; and it seems very probable that the primary movement of 

 these northern faunas was from the same African dispersion area as 

 that of the south. 



