FAUNAL PROVINCES AND REALMS 347 



Now we come to the most extensive of all Devonian faunal realms, the 

 Euro-Asiatic one of Upper Devonian time. This is at first characterized 

 by the bracliiopods Stringoceplialus burtini, liijpothyris, and Gypidula, 

 and by a series of goniatites. These Upper Devonian assemblages have a 

 common expression throughout the Northern Hemisphere, with the ex- 

 ception of America roughly east of the Mississippi Eiver. Here the Mid- 

 dle Devonian Hamilton fauna, somewhat modified, persists into the 

 Ithaca biota of early Upper Devonian time, but soon this marine region 

 goes over into a great bay with sluggish waters depositing black shales to 

 the west of the great sandstone delta of the Appalachian geosyncline. 

 But, even under these adverse conditions, there are received pulsations of 

 the Ilypothyris cuboides and Spirif&i^ disjunctus faunas, which appear, 

 however, to have come from western Europe down the Saint Lawrence 

 portal, battling for possession of the Appalachian sea east of the Ozark- 

 Kankakee axis. On the other hand, the more typical Euro-Asiatic faunas 

 spread from the Arctic down the Mackenzie Valley into Minnesota, Iowa, 

 and Michigan, and during short intervals the Spirifer hungerfordi faunas 

 were able to penetrate into central New York. 



Summation 



In summary, we see that the faunal provinces and realms are marked 

 at times by identical species of very wide distribution, but more often by 

 generic and family developments spreading throughout the shallow sea- 

 ways for some thousands of miles, or as far as they can find a similar 

 environment. These entities are prevented from spreading everywhere, 

 not so much by temperature, but mainly by barrier lands and by deep 

 oceans of wide extent. Even so, some of the ground-attached, shallow- 

 water invertebrates succeed in establishing themselves directly across the 

 oceans ; but this is possible only among those that have long-lived floating 

 larvae, which are several weeks in developing before they are compelled 

 to seek the marine bottoms. As a rule, the larv^ of bottom-living inver- 

 tebrates are free-floating and are carried about by the currents for only a 

 few days ; consequently but few of them- are able to spread and mature 

 beyond the range of the shallow seas of their birth. 



From the evidence recited it will also be seen that provinces and realms 

 are characterized now by one and now by another class of invertebrates 

 or by a group of them. At one time the dominating entities are of the 

 attached bottom life, like the brachiopods; at another they are the slug- 

 gishly crawling kinds of gastropods and bivalves, and at yet other times 

 the more active trilobites or cephalopods. And so it has always been : 



XXIV— Bull. Geol. Soc. Aji., Vol. 32, 1920 



