472 GEOLOGY. 



indeed their climax of deployment lies somewhere here. Their curve 

 was so broad that its highest point is not easily fixed with exactness. 

 In many respects the brachiopods of this fauna were closely allied 

 to those of the Onondaga and many species passed unchanged from 

 the earlier into the later fauna. In most cases the changes in the 

 environment were paralleled by changes in form that rose to specific 

 value. The lowly inarticulate brachiopods still persisted as a minor 

 factor, while the more highly differentiated forms attained wide diver- 

 sity and high rank. The stropheodontas retained about the same 

 relative place, waning somewhat perhaps in size, though still often 

 large. The spirifers reached a climax in the extension of their hinge- 

 lines (Fig. 210, i). This extension is particularly characteristic of 

 the Hamilton epoch, though a feature of the Devonian generally. 

 The geological series perhaps presents no simpler or more obtrusive 

 character by which a formation may be identified than these excessively 

 broad-winged spirifers. Some specimens had a breadth along the hinge- 

 line four or five times as great as the length of their shells, if breadth 

 may be said to be greater than length in any case. The Orthis family, 

 once so prominent, had fallen to a very subordinate place. The pro- 

 ductids increased notably, and were obviously advancing toward 

 their great expansion in the Carboniferous period. The meristoids 

 fell away and were nearing the close of their career. The loop-bearers 

 of the Terebratula and Cryptonella types increased. The persistent 

 Rhynchonella family was represented by species wonderfully prolific 

 in individuals. The pentamerids made but a feeble showing. Several 

 species of extreme interest and significance invite special notice, among 

 which are Tropidoleptus carinatus (Fig. 210, k), Vitulina pustulosa 

 (Fig. 210, I), and Chonetes coronatus (Fig. 210, m and n). The first 

 two belong to peculiar genera not previously found in the interior 

 basin. Apparently they could not have been derived from any of 

 the preceding inhabitants of the region. The third species belonged 

 to a genus represented by other species that previously lived in the 

 region, but this particular species is peculiar, and does not give signs 

 of any near kinship to the previously resident members of the genus. 

 All these forms were locally abundant in the Hamilton assemblage. 

 They bear the marks of immigrants, however, and a search for their 

 former home and relatives reveals the fact that in South America 

 the same species, or closely allied ones, lived in abundance, and that 



