388 PALiEONTOLOGY OF NEW-YORK. 



" Wherever these large crustaceans are found, and with them small Lin- 

 '• gulae and other fossils, we may be sure that we are at or near the very 

 " summit of all rocks to which the term Silurian can be applied, and that 

 " the next overlying stratum belongs to the first great era of fishes, the 

 " Devonian, or Old Red standstone ; for the thin transition band now 

 " under consideration still remains what I stated it to be twenty-one 

 " years ago, the lowest in which the trace of true vertebrated animals 

 ** has been detected" *. 



Sir lloDERiCK has shown that a similar group of crustaceans occupy a 

 parallel position in the uppermost Silurian beds near Lesraahago in Scot- 

 land*; and from facts elsewhere observed, infers that gigantic crustaceans 

 of the same genera mark the uppermost Silurian zone everywhere in the 

 northern hemisphere. He further says : " Near Ludlow, Hereford and 

 '• several other places, the thin course with small fish-bones has been 

 " traced over an extensive area ; and in several places where the fishes 

 " are wanting, the band is still characterized by the associated large 

 " crustaceans"! . 



In this country, however, the line of demarcation between the strata 

 containing these crustaceans, and those containing the fishes, is very 

 clearly marked, even where the two sets of strata are in contact ; the 

 lower or crustacean beds being always an argillaceous magnesian lime- 

 stone, while the higher beds, or lowest fish-bearing beds, are of gray or 

 bluish gray limestone, which, however, in some localities, becomes of a 

 drab color, but never loses its distinctive features, or assumes the cha- 

 racter of the rock below. 



In New-York, and so far as we know in the United States, the first 

 appearance of these peculiar crustaceans is at the epoch of the Clinton 

 group, where some fragments of spines, apparently of Ceratiocaris, are 

 known to occur. Well-marked spines of Ceratiocaris (one of which is six 

 inches long, the C. deweyi) have been found in the Niagara group, and 

 species of this genus occur with Euryptcrus in the Waterlime group ; 

 while from strata above this horizon, no remains of similar character have 



* Qnarterly Geological Jonraal, Vol.xli. p 24. t Id. p. 28. 



