THE DEVONIAN PERIOD. 475 



gradually extended itself to the southward and at length to have joined 

 the eastern sea across the tract stretching from the old lands of Wis- 

 consin to the similar old lands of Missouri which had in the earlier 

 stages formed a separating isthmus, but which at length became sub- 

 merged by the advance of the seas on either hand. This northwestern 

 fauna was closely allied to the Middle Devonian fauna of eastern and 

 central Europe. So close indeed is the similarity that the inference 

 is forced that there was free intercommunication between the two 

 regions. The prolonged sea-arm that reached down the great interior 

 valley of the northwest was perhaps no more than an eastern exten- 

 sion of the Eurasian Middle Devonian province. 1 The extension of 

 this great sea-arm took place late in the Devonian period, for the 

 strata bearing its peculiar life lie directly on the older formations in 

 Missouri, Iowa, and Minnesota, with no Lower Devonian beds between, 

 showing that the sea crept in after the earlier Devonian epochs had 

 passed. The brachiopods furnish the most decisive evidence respect- 

 ing the relations and genesis of this northwestern fauna. 



Brachiopods as evidence of geographic connections. — In Europe there 

 is a horizon so well marked by Hypothyris (Rhynchonella) cuboides 

 that it is known as the " cuboides zone " and has a notable place in 

 the literature of the subject under that name. 2 This species also occurs 

 in eastern Asia (China), in northwestern America (Mackenzie valley), 

 in western America (Great Basin), and in the American interior, as 

 far east as New York. To show how well the peculiarities of this 

 form are held through this wide distribution, figures of typical indi- 

 viduals from England (Fig. 212, h), Russia (Fig. 212, /), China (Fig. 212, 

 g), the Great Basin (Fig. 212, i), and New York (Fig. 212, e) are 

 arranged side by side. Their close similarity is very notable. Spiri- 

 fer disjunctus is another brachiopod of similar range, illustrations of 

 which from England, China, Mackenzie valley, and New York are 

 arranged for comparison (Fig. 212, a, b, c, and d). Schizophoria (Orthis) 

 striatula is still another species common to Europe, Asia, and different 

 parts of America, and is illustrated by specimens from England, China, 

 Mackenzie valley, Nevada, Iowa, and New York (Fig. 212, /, k, l } 

 m, n, and o). Gypidula comis from Iowa and China is illustrated by 

 Fig. 212, p and q, and Spirifer tullius by specimens from New York 



1 Weller, Journ. of Geol., Vol. VI, p. 306. 



2 Williams, The Cuboides Zone and its Fauna. Bull. G. S. A., Vol. I, p. 481, 1890. 



