352 ORTMANN — DISTRIBUTION OF DECAPODS [Aprils, 



perate parts of South America, from the Pacific to the Atlantic 

 Ocean, but do not extend into the tropical parts. 



In order to arrive at an understanding of these points it will be 

 necessary to investigate the history of the origin and the mutual 

 relations of North, Central and South America. It is generally 

 conceded that these three parts have undergone various changes, 

 but as regards the details there is much discussion and erroneous 

 ideas prevail. It will hardly be possible in the following to give 

 satisfactory answers to all of these questions, but we shall endeavor 

 to collect all that is known as relating to the geological history, 

 and we shall thus try to get an idea of the most prominent features 

 of the history of the origin of the Americas. 1 



That America consists of three masses differing tectonically is 

 well known. The nucleus of North America is formed by an old 

 northern and eastern mass — the " Canadian shield" and the folded 

 ranges to the south of it. The parts to the west of these were sub- 

 ject to various oscillations during Paleozoic and Mesozoic times, 

 and finally the elevation of the chain of the Rocky Mountains, 

 running in a north-southerly direction, resulted in the present con- 

 figuration of North America. 



Central America (including the northern coast of South America) 

 consisted in older times of a system of old ranges with east-westerly 

 strike, forming probably an old Paleozoic and Mesozoic continent 

 (Antillean continent), which was destroyed at the end of the Meso- 

 zoic time. Since then Central America and the West Indies are 

 composed only of the remnants of this continent, which in turn 

 have undergone various changes. 



South America consisted formerly of the old Brazilian plateau, 

 which probably was part of old Gondwana Land (Australia, Africa, 

 South America). The high moumain chains of the Cordilleras in 

 the west did not exist for a long time, and this region was covered 

 by sea probably up to near the end of the Mesozoic time. The 

 elevation of the Cordilleras began at the end of the Cretaceous and 

 continued during the Tertiary. 



The present connection of the three Americas did not always 



1 We shall disregard all those questions which are not connected with and 

 illustrated by the distribution of the freshwater Decapods — for instance, the sup- 

 posed former connection of North and Northeast America with Europe. 



