HUDSON-RIVER GROUP. 
289 
Position and locality. Near Washingtonville, Oswego county, in the central part of 
the group, and in transported fragments in all the southern counties of the State ; often 
abundant. ( state Collection.) 
342. 20. ORTHIS CENTRILINEATA (n.sp.). 
Pl. LXXIX. Figs. 5 a, b, c. 
Semioval; length and breadth nearly equal; dorsal valve moderately convex; stria? 
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 striae are more numerous and often trifid, and the 
dorsal valve less convex. It is readily distinguished from 0. testudinaria , with which it is 
associated, by the slightly elevated convex valve, and the stronger radiating striae, which 
show no evidence of concentric striae. 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 c. Several striae enlarged, showing the bifid and trifid character. 
Position and locality. This species is found associated with Orthis testudinaria , Trinucleus, 
and crinoidal columns, in the higher part of the group, at Loraine, Jefferson county, and 
at Turin in Lewis county. ( State Collection.) 
186. 19. ATRYPA INCREBESCENS. 
Pl. LXXIX. Fig. 6. 
Reference pag. 146, pl, 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 Atrypa modcsta ( 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 Hudson-river group. They are, 
however, by no means characteristic of the rocks of this formation. 
| Palaeontology.] 37 
