146 
PALAEONTOLOGY OF NEW-YORK, 
Fig. 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 (n. sp.). 
Pl. XXXIII. Figs. 13 a-y. 
Compare Terebratula plicatella, Dalman ( Anomia plicatella, Linnjeus), 1827, Vet. Acad. Handl. pag. 56, 
pi. 6, fig. 2. 
— — Hisinger, 1837, Lethsa Suecica, pag. 80, pl. 23, fig. 4 a, b, c. 
Spheroidal, gibbous, ovoid or subtriangular ; length and breadth 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 ( Sowerby, 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 (Sowerby, Sil. System , pag. 641, pl. 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 , b , c and d. 
In fig. 13 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 
