226 paljEontology of new-yokk. 



staled ihus : 'J'linl all iliose loniis requiring ciilcareoiis sediment for their full development, 

 will flourish during the deposition of such material, but become diminished or entirely 

 exterminated when a change to argillaceous or arenaceous deposits takes place. On the 

 other hand, those forms which require a very small proportion of calcareous matter, and 

 flourish in the argillaceous mud, are diminished or cease altogether when a calcareous 

 deposition supervenes. The forms which maintain a bare existence through a series of 

 calcareous deposits, become extensively developed so soon as the nature of the sediment 

 changes ; and tlie same may be said of those requiring calcareous sediment, during a period 

 of argillaceous deposits. 



Those changes in the nature of the sediment, which may afTcct the majority of species 

 in the way we have mentioned, will, in others, produce a total destruction or extermina- 

 tion, because they are not adapted to encounter such extreme changes. This is, in a great 

 degree, true of the Trilobites. Of the sjjecies known in the Trenton limestone of New- 

 York, scarcely one fourth are found in the shaly strata which succeed ; and, with two 

 exceptions (the Calymcne and Trinndeus) , those which are known are extremely rare. 



In a case like the present, where the higher shaly part of the formation much exceeds 

 the lower calcareous part, reaching the thickness of nearly one thousand feet, we are very 

 likely to lose sight of the characteristic fossils of the lower division of the group, and to 

 regard them as of little or no importance in the identification of the higher strata. Neither 

 are they, while the nature of the deposit continues uniformly argillaceous ; but so soon as 

 the calcareous matter is increased, we find, spontaneously as it were, the appearance of 

 forms vviiich we have l)efore known in the lower part of the formation. We may recollect, 

 however, that not only are certain families affected by this change in the sediment, but 

 different species of the same family are differently affected. In the present instance, the 

 Triarthrus and Trinudeus become more abundant in the shaly portion of the strata, and 

 we find two other forms which have not been seen in the calcareous part of the formation. 



Nearly all the characteristic genera of Trilobites appear at once during this period ; 

 and all the subsequent forms in our strata are referable to these, or some modification of 

 them. Some of them, as Phacops and Calymene, are continued throughout the Silurian 

 and Devonian rocks, with scarcely any modification of form in some species, while others 

 present a wide departure from the original type. The Platynoius, Illtcnus* and Jlcidaspis, 

 reappear in the Upper Silurian strata; while Trinudeus, Ccraurus, Isotdus, Jlsaphus and 

 Ogygia, arc unknown beyond ihe strata of (his period. 



* I rel'er here to Sumastis barriensis of the Niagara strata, vvliicli, tliougli it may conslitiitc a distinct genus, is 

 nevertheless, constructed as the true Illanus in its important part-;. 



