In Silurian and Devonian Epochs 107 



shallow expanses were invaded at periods by the sea. The 

 waters were then isolated and their salt precipitated. 



Before reviewing the above evidence, it should be stat- 

 ed that supposed fish remains have been reported from the 

 base of the Upper, or even from beds of the lower Silurian, 

 but the records are so scant, or the material is so doubtful 

 that we prefer not to found any views upon such. There 

 is no reason, however, why such records need not be verified 

 and even greatly extended in the future, for the most prim- 

 itive fishes almost certainly existed when the oldest Silurian 

 rocks were deposited. 



Mention should here be made of an extremely abundant 

 type of organic remain that is scattered over many layers 

 of strata from Lower Silurian up to at least Carboniferous 

 days. For the small or almost minute brown to nearly 

 black shining bodies that have been named "conodonts" 

 or cone teeth, have attracted the attention of observers in 

 Russia, Britain, and not least in N. America. These in 

 some cases are simple cone-like teeth, which, when attached 

 to the animal forming them, were probably composed in 

 part at least of carbonate of lime, but may also have been in 

 part horny. But most show a compound structure, and con- 

 sist then of a tooth-plate that bears from two to many teeth 

 of varying size, number, and disposition. 



Newberry, Hinde, and others have well compared these 

 with the teeth and tooth-plates of living Cyclostome fishes, 

 which however are wholly or largely of a horny nature. The 

 writer also has suggested that they may all represent evolv- 

 ed and complex derivatives from the horny teeth formed in 

 the mid part of the proboscis in Metanemerteans. For if we 

 accept that freshwater Metanemerteans may have swarmed 

 in Cambrian lakes and swamps, these by progressive change 

 may have given rise to Cyclostomes, some of which retained 

 horny teeth as in existing types, while others may have ad- 

 vanced to a more complex calcareous type. 



The writer has arranged in Fig. 7, a set of illustrations 

 from a to r, some of which are the horny teeth of existing 

 Cyclostome fishes, and others are conodonts copied from 

 the works of Newberry and Hinde. The close resemblance 

 in many cases is at least suggestive. The writer also has 



