HUDSON-RIVER GROUP. 289 



Position and locality. Near Washingtonville, Oswego county, in the central part of 

 the g^roiip, and in transported fragments in all the southern counties of the State ; often 

 abundant. {State ColUttUm.) 



342. 20. ORTHIS CENTRILINEATA (h. syj.). 



Pl. LXXIX. Figs. 5 a, b, c. 



Semioval ; length and breadth nearly equal ; dorsal valve moderately convex; stri* 

 about thirty, bifid or trifid towards the margin ; cast of the visceral impression bilobate, 

 small, narrow, sublinear, with a depressed line extending from the centre towards the 

 base of the shell. 



The specimens of this species yet seen are casts, with a small portion of the shell 

 remaining. It approaches more nearly to 0. dichotoma of the Trenton limestone, than to 

 any other species known to me; but the strias are more numerous and often trifid, and the 

 dorsal valve less convex. It is readily distinguished from 0. tcstudinaria, with which it is 

 associated, by the slightly elevated convex valve, and the stronger radiating stria;, which 

 show no evidence of concentric striaj. It bears some resemblance to Orthis vespertilio of 

 MuRCHisoN, but is uniformly smaller, and evidently distinct. 



Fig. 5 a. A specimen of the natural size. 



Fig. 5 b. A magnified portion of the cardinal line, and visceral impression. ; 



Fig. 5 e. Several striae enlarged, showing the bifid and tiifid character. 



Position and locality. This species is found associated with Orthis tcstudinaria, Trinucleus, 

 and crinoidal colunms, in the higher part of the group, at Lorainc, Jefferson county, and 

 at Turin in Lewis county. {State Collection.) 



186. 19. ATRYPA INCREBESCENS. 



Pl. LXXIX. Fig. C. _ ' . _ 



Refeience pag. 146, pi. 33, figs. 13 a - y, of this volume. 



This species is of frequent occurrence in the lower part of the Hudson-river group, 

 rarely attaining a larger size than the figure given. It is always crushed, and is not readily 

 recognized as identical with that of the Trenton limestone. 



Position and locality. In the Utica slate, at Turin, Lewis county, and in the shales of 

 the Hudson-river group at the same place, and at Loraine. {State Collection.) 



In addition to the preceding forms, the Mrypa modesta ( see page 141 ) occurs in the 

 upper part of the Utica slate ; and very rarely fragments or casts of some other Trenton 

 limestone species are found, either in this slate or in the Iludson-river group. They are, 

 however, by no means characteristic of the rocks of this formation. 



I P.tL.EONTOLOGY.] 37- 



