226 PAUBONTOLOOY OF NEW-YOKK. 



Stated thus : TIml all those forms requiring calcareous sediment for their full development, 

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

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

 other hand, tliose 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 

 clianges ; and the 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 aflfect 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 species known in the Trenton limestone of New- 

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

 exceptions (the Calymene and Trinucleus) , 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 which we have before 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 Trinucleus 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 Platynotus, Illamus* and Acidaspis, 

 reappear in tiie Upper Silurian strata ; while Trinucleus, Ceraurus, Isotelus, Asaphus and 

 Ogygia, are unknown beyond the strata of this period. 



* I refer here to Bumastu barrieruU of the Niagara strata, which, though it may constitute a distinct genus, is 

 UevertheleM, constructed as the true lUcmut in its important part^. 



