78 



while gastropods and pelecypods became common. The 

 Plenrodictyutn still continued to exist in great numbers for 

 some time, but finally it too disappeared. After that, the 

 sea was peopled chiefly by brachiopods and bryozoa, while 

 trilobites continued to increase in number. Before long, a 

 rich brachiopod fauna had developed, and the deposition of 

 the sediment went on at a somewhat accelerated pace. 

 Another change occurred, and the rich brachiopod fauna 

 disappeared. With this was brought to a close the deposi- 

 tion of the fossiliferous shales underlying the Trilobite beds. 

 Then came a time of more shallow water, when grittv 

 sediments were deposited along with the lime and mud. 

 Here trilobites found conditions more favorable to their 

 existence, and for a long time they continued to people the 

 waters in great numbers. Occasionally they were kept 

 down for a time, by unfavorable changes, and in their stead, 

 other forms, such as Athyris and Streptelasma flourished. 

 Eventually, however, they regained their vitality and again 

 increased rapidly. 



Then there came a time when the muddy waters allowed 

 but little development of life, so that the shales succeeding 

 the Trilobite beds are comparatively barren. When, how- 

 ever, the conditions recurred which favored the great 

 development of the trilobites, these latter did not return. 

 Probably by this time the great Placoderms had found their 

 way into these waters in considerable numbers, and to them 

 may be due the destruction of the trilobites. It is known 

 from the specimen ( before referred to ) discovered by Mr. 

 Myer, that these fishes were present at that time, and as 

 they were ground feeders, the supposition lies near that they 

 preyed upon the crawling trilobites and other bottom 

 lorms, and thus kept the water comparatively free from 

 invertebrate life. 



For a long time after this, animal life was scarce in the 

 Hamilton sea of this region, brachiopods alone occurring in 

 anything like abundance. Then came the arrival of number- 



