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in the formation of that bed. The species associated with 

 Nautilus magister, practically shared its fate, none of the 

 characteristic forms of the Nautilus bed appearing in any 

 higher horizon, with the exception of Pleurodictyum stylq- 

 pora, and perhaps a few straggling individuals of Camaro- 

 toechia clotis. Pleurodictyum stylopora is another example 

 of a species which became extinct in this region shortly after 

 the opening of the Hamilton conditions, in the early times of 

 which it flourished in great numbers.* Many of the species 

 which lived at the beginning of the Hamilton epoch in this 

 region, (many of which had undoubtedly immigrated from 

 other localities ) disappeared after the cessation of conditions 

 favorable to their existence. While the mature individuals 

 died, their more adaptable offspring migrated by successive 

 stages, and finallv became established in other localities. 

 Towards the close of the Hamilton epoch, however, when 

 the conditions favorable to the existence of the species again 

 appeared, the species slowly returned. We find, therefore, 

 the upper beds of the Hamilton shales, i. e. the Demissa and 

 Stictopora beds, filled with species which are common in the 

 Pleurodictyum beds and the shales just above. 



It is not to be presumed, that during the time-interval 

 w T hich elapsed between the deposition of the lower and upper 

 Hamilton beds, the migrated faunas made no attempt to 

 return. The fact that such attempts were actually made 

 seems to be indicated by the few straggling representatives 

 of these faunas, such as Tropidoleptus carinatus and others, 

 which occur at intervals in the beds overlying those in 

 which they make their first appearance. But while a few 

 individuals may have existed, the species never multiplied, 

 until the time when the associated species which existed side 

 by side with it during the earlier ages, returned. The causes 

 for this may have been manifold, such as lack of proper food, 



*It must hi- noted, however, that this species occurs in higher horizons farther 

 east, and this may indicate an eastward migration alter the conditions had become 

 unfavorable in the west. On the other hand the eastern representatives of this 

 species in the Moscow shale may have conic from the same favorable locality from 

 which the western forms had migrated at the Opening of the Hamilton epoch, the 



species becoming entirely extinct in this region at the end of that time. 



