146 PALEONTOLOGY OF NEW^YORK. 



Fi"' 12 a. View of the ventral valve, with the beak of the dorsal valve projecting above. 



Fig- 12 b. Profile view of the same. 



Fig. 12 c. Front, with the dorsal valve below. 



Position and locality. In the upper part of the Trenton limestone at Turin, Lewis county. 



{^Collection of Mr. Alson Clarke, of Turin, on whose authority this species is quoted from the Trenton 

 limestone.) 



186. 19. ATRYPA INCREBESCENS {»..?;).). 



Pl. XXXIII. Figs. 1.3 a-y. 



rompare Terchrattda plicatclla, D.^lman {Anomia plicatella, LiNNi5--.us), 18-37, Vet. Acad. Handl. pag. .56, 



pi. 6, fig. 2. 

 — — HisiNGER, 1837, Lethsea Suecica, pag. 80, pi. 23, fig. 4 a, b, c. 



Spheroidal, gibbous, ovoid or subtriangular ; length and lireadth nearly the same, in 

 older specimens very variable ; dorsal valve with a broad, more or less deep sinus, which, 

 in older specimens, reaches nearly to the beak, and is marked by three or four plications, 

 which finally become much elevated in front, producing a deep emargination in the opposite 

 valve ; beak of the dorsal valve small, acute, and, in young specimens, slightly incurved, 

 with a perforation beneath the apex. In older shells, the beak becomes strongly incurved, 

 and closely pressed against the opposite valve. Ventral valve regularly convex in young 

 shells, with a slight elevation in front ; as the shell advances in age, the medial lobe is 

 developed, and, in old specimens, reaches nearly to the beak. Surface of the shell marked 

 by twelve to sixteen plications, three or four of which are in the sinus and four or five on 

 the medial lobe, the plaits never subdivided ; transversely marked by elevated flexuous 

 imbricating lines, which, in young and well preserved specimens, continue entirely to the 

 beak. 



This species is an associate of Delthyris lynx, and is equally variable with that shell. It 

 has been referred to Terebratula tripartita ( Sowekby, Sil. System, Vol. ii. fig. 15), a 

 Caradoc species ; but I regard our shell as quite distinct from that one. The smaller 

 varieties approach more nearly to T. pusilla (Sovverbv, Sil. System, pag. 641, pi. 21, 

 fig. 18). If described at all, in any foreign work within my reach, the species does not 

 present the remarkable variation in form, on the other side of the Atlantic, that it does in 

 this country. 



This species, like its constant associate Delthyris lynx, is much more developed in the 

 West than in New-York, where all the specimens I have seen are small, and rarely 

 approach the globose forms of western localities. The most common forms in New- York 

 are those represented in figs. 13 a, h, c and d. 



In fig. 12 a, b, ventral valve and front view, the specimen has 16 or 17 distinct plications, four of which 

 are elevated upon the mesial lobe. The form of the shell is somewhat triangular, the mesial 



