234 
PALAEONTOLOGY OF NEW-YORK. 
the middle of the valve; while three or four of those on the middle of 
the ventral valve are depressed towards the front into a sinus, which is 
faint and broad in some specimens, and narrow and more distinctly 
defined in others. A few strong imbricating zigzag lines of growth near 
the margins of the valves. 
This species is quite rare, and the individuals figured may be only the young, 
the mature forms having escaped observation. 
Fig. 7 a, b. Ventral and profile views of a small individual. 
Fig. 8 a, b. Ventral and front views of a more rotund form. 
Geological position and locality. In the shaly limestone of the Lower Helderberg 
group : Albany county. 
miynchonella transyersa (n. s.). 
Plate XXXIV. Fig. 9-16. 
Shell subtriangular, wider than long, tapering abruptly to the beak. 
Ventral valve depressed convex, most prominent near the beak : beak 
arched. Dorsal valve a little larger, most elevated near the front : beak 
incurved; foramen narrow, continued up to the apex of the beak. 
Surface ornamented by about fourteen or fifteen sharply elevated plica¬ 
tions on each valve, of which from three to four are elevated near the 
front into a rather faint mesial fold, and two or three depressed on the 
ventral valve so as to form a faint sinus in the front. Somewhat strong 
zigzag lines of growth mark the surface of the valves near the margin 
in front. 
Fig. 9 - 14 a, b, c. A series of individuals showing a regular gradation in size, with a 
gradually increasing depth of sinus. 
Fig. 15 & 16 a, b , c. Individuals having a more strongly defined sinus and mesial lobe ; a 
single individual having but two folds in the sinus. 
There is usually associated with this species another which agrees very nearly 
with it in character, being wider and more depressed in form, and which, without 
a better series of specimens for examination, I am unwilling to regard as distinct 
from this one. See figs. 17 - 19, same plate. 
Geological position and locality. In the shaly limestone of the Lower Helderberg 
group : Helderberg mountains, Albany county. 
