PALAEONTOLOGY OF NEW-YORK. 
226 
stated thus : That all those forms requiring calcareous 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 the same maybe said of those requiring calcareous sediment, during a period 
of argillaceous deposits. 
Those changes in the nature of the sediment, which may affect 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- 
Yorlc, scarcely one fourth are found in the 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, Illcenus* and Acidcispis , 
reappear in the Upper Silurian strata; while Trinucleus , Ccraurus, Isotelus , Asaphus and 
Ogygia, are unknown beyond the strata of this period. 
* I refer here to Bmnastis barriensis of the Niagara strata, which, though it may constitute a distinct genus, is 
nevertheless, constructed as the true Illcenus in its important parts. 
