77 



insufficiency of calcareoUvS material in the water, increased or 

 diminished depth or temperature, unfavorable currents, or 

 other injurious physical conditions. The existence of other 

 creatures, \yhich preyed upon the species, may also haye 

 been the cause of the paucity of individuals . In this latter 

 case the reappearance of the species in great numbers in the 

 later beds, ma}' have been due to reinforcement of the 

 survivors from without, by the immigration of numerous 

 individuals, so that by sheer force of number the\' survived 

 the wholesale destruction, which formerly kept them down. 



The sequence of changes during the continuance of the 

 early Hamilton epoch was something as follows : The 

 epoch opened with the formation of the Nautilus bed, the 

 water being comparatively pure and free from coarse 

 sediment. On the floor of the shallow ocean, grew millions 

 of tiny brachiopods, which in places were closeh' packed, 

 growing over and on each other and making clusters, in 

 appearance recalling those of our modern mussel beds. Over 

 these beds crawled the gi-eat Nautili, wnth their coiled and 

 probably highh'-colored shells. Brachiopods probably 

 formed the food of these creatures, though many soft-bodied 

 and shellless animals undoubtedW existed, of which no trace 

 has been preserved. Here and there may have appeared a 

 Nautilus floating on the surface or swimming vigoroush' to 

 escape some hungry shark, which had wandered into the 

 region. Scattered over the sea floor among the brachiopods 

 were the tiny coral heads of the PleurocHctyum, the polyps 

 probably with brilliant colors. Trilobites crawled about, 

 but they were not ver\' common. Gastropods likewise 

 occurred, as well as a few pteropods, but pelecypods seem to 

 have been absent altogether. With the beginning of the 

 second Pleurodictyum bed the conditions changed. The 

 Nautilus became extinct, the waters probably shoaled some- 

 what, and brachiopods continued to increase in number and 

 variet}', the earlier species, however, having disappeared. 

 Brj^ozoa began to grow, and often formed large fronds, 



