10 
PALEONTOLOGY OF NE'W-YORK. 
390. 23: ATRYPA PLICATA (h. sp.) 
Pl. IV. Fig. 6. 
Smaller valve suborbicular, wider than long ; surface marked by 12 sharp plications, without 
any perceptible mesial elevation ; plications marked, in the shell, by fine concentric lines ; beak 
small, incurved. 
The specimen is the cast of a single' smaller valve, but it presents characters unlike any of 
the preceding species. In its form and plications it resembles the A. increbescens, but differs in' 
having no mesial elevation, and the form of the cast at the beak (which is not well represented 
in the figure) differs essentially from that shell. 
Position and locality. This shell occurs with the following species, in the higher red portions 
of the rock at Lockport. (Collection of Col. Jewett.) 
391. 17. MODIOLOPSIS ORTHONOTA. 
Pi.. IV. A. Figs. 1 a, b, c. 
Unio orthonofus. Conrad, Annual Report of 1839, p. 66. 
Cypricardia orthonota. ■ Hall, Geol. Report 4th Dist. New-York, 1843, pag. 48, figs. 8 & 9. 
Subelliptical or rhomboidal, obliquely truncated before ; hinge-line straight; posterior 
extremity rounded or obliquely truncated ; sides nearly parallel; umbones elevated, thin, sharp, 
with a defined ridge extending to the posterior basal margin ; anterior to the ridge is a flattened 
space widening from the umbones towards the base, anterior to which the shell is a little more 
ventricose ; muscular impression at the anterior extremity distinct, no other Visible ; surface 
marked by fine concentric lines. 
The specimens of this species are usually casts, preserving some faint concentric lines, the 
markings of the shell. There is little difficulty in identifying this species by its general form. 
The characters are not always so clearly defined as in the larger specimen figured ; but in all 
the variety of smaller forms, there is not a sufficient departure from those figured as to mislead. 
Some of the smaller specimens bear a close resemblance to M.faba ( Palaeontology , Yol. i. 
pag. 158, pl. xxxv. fig. 6 a, b, c) ; but the hinge-line is much more straight, and the posterior 
extremity usually less rounded ; and the sinus, which is so conspicuous in M.faba, is only 
represented by a flattened or slightly depressed space. 
This species is quite abundant in some of the higher layers of the rock. 
Fig. 1 a. Right valve of a small specimen, with the posterior extremity rounded' and the anterior 
one extended. 
Fig. 1 b. Right valve of a larger specimen, having the anterior extremity less prolonged. 
Fig. 1 c. View of the hinge-line and cardinal area of the last. 
Position and locality. This shell is usually found about fifty or sixty feet below the top of 
the sandstone at Medina, and in the higher part of the rock at Lockport. (State Collection.) 
