THE DEVONIAN PERIOD 585 



epoch, and subordinately of the Devonian generally. Some 

 specimens had the breadth along the hinge-line four or five times 

 as great as the length of their shells. The Orthis family, once so 

 prominent, had fallen to a lower place, while the moductids made 

 notable advances toward their great expansion in the Carbonif- 

 erous period (Fig. 419). Some of the brachiopod species were 

 closely related to species that lived in South America just before 

 their appearance in the North American fauna, which supports the 

 view that they were immigrants from the south. The muddy 

 bottoms favored the mollusks. The goniatites increased in numbers 

 and size (Fig. 419). Some of the species began to exhibit node-like 

 ornamental expansions of the shell which became a conspicuous 

 feature later. Some of the primitive straight cephalopods still 

 persisted. The pelecypods especially were favored by the muddy 

 bottoms and the number of recognized species approaches 200. 

 A part of these were derived from Onondagan predecessors, while 

 others were peculiarly Hamilton types, being immigrants or forms 

 whose parentage is unknown. The gastropods were inferior to the 

 pelecypods in number. Their genera were mostly Onondagan, 

 though the species were largely new. The Hamilton trilobites were 

 inferior to the Onondagan in numbers and ornamentation. Other 

 crustaceans showed some increase of representation. At this time 

 appeared the first known barnacles of the northern sessile type. In 

 losing its pedicel and in fixing itself immovably on other objects 

 it became degenerate, but it found a lowly place to which it has 

 hung with wonderful persistence, not unlike the debased human 

 class which it has come to typify. 



The Northwestern Hamilton fauna. While the preceding 

 fauna was developing in the eastern interior sea, another fauna was 

 evolving on somewhat different lines in a sea which, advancing 

 from the northwest, appears to have overspread a large tract of the 

 northwestern interior. This sea embraced a portion of the Mac- 

 kenzie basin and extended southward through Manitoba to Iowa 

 and Missouri. For a time this northwest sea was not in communi- 

 cation with the sea in which the Southern Hamilton fauna lived 

 (Fig. 409), but it finally crossed the intervening barrier and its 

 fauna overran the territory already occupied by the Southern 



