250 pai,;f.ontology of new-york. 



TRILOBITES OF THE UTICA SLATE AND HUDSON-RIVER GROUP. 



Plate LXVI. ( in part ). 



The important and characteristic Trilobites of these rocks are species which appear, 

 though less conspicuously, in the Trenton limestone, and have been already noticed. The 

 rule I have adopted, however, renders it necessary to speak of them here, giving- illustra- 

 tions of specimens from these rocks. The most numerous forms are those of Calymene 

 beckii and Trinucleus concentricus ; and the condition of the specimens in the shaly strata is 

 so different from the same in the compact limestone, that both require illustration. Two or 

 three forms appear in these rocks, which are thus far unknown in the limestones below, 

 and which are probably peculiar to this period, becoming developed only after the com- 

 mencement of argillaceous deposits. 



297. 2. CALYMENE BECKII. 



Pl, LXVI. Figs. 2 a-k; and Pl. LXVII. Figs. 4 a, b, c, d, e. 

 Reference pag. 2^1, pl. 64 of this report. 



Much confusion has arisen respecting this species, which is abundant in the Utica slate. 

 It was first described by Prof. Eaton, under the name Brongniartia, from imperfect 

 specimens of the cephalic shield, in the calcareous beds of the Utica slate, at Coldspring, 

 Montgomery county. These specimens are more convex than those in the softer slate, and 

 their true character is not obvious on a cursory examination. Prof. Green substituted the 

 name of Triarthrus for that proposed by Prof. Eaton ; still regiyding the small imperfect 

 cephalic shields as the entire animal. Dr. Harlan, in correcting this error of Eaton and 

 Green, described the buckler as destitute of oculiform tubercles ; and I have, myself, 

 fallen into the same error in describing as distinct species two nearly entire specimens, 

 owing to the absence of the maxillary shields in one, and their partial preservation in the 

 other.* 



* Silliman's Am. Jour. Science, Vol. xxxiii, p. 137. 



