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PALAEONTOLOGY OF NEW-YORK. 
Spirifera angusta. 
PLATE XXSYIII A. 
Spiri/er angusta : Hall, Tenth Report on State Cabinet, p. 164. 
Shell very depressed pyramidal, extremely extended transversely; car¬ 
dinal extremities attenuate, acuminate: valves very unequally convex, 
with surface finely plicate. 
Yentral valve proportionally very deep, forming nearly the entire thick¬ 
ness of the shell : beak elevated and pointed, or slightly incurved at 
the apex; the cardinal margins sloping very regularly to the hinge- 
extremities. Area extremely elevated and inclined forward; the full 
height equal to the length from the beak to the front of the valve; 
longitudinally and vertically striate, divided in the middle by a fora¬ 
men which is but little more than half as wide as high, and distinctly 
grooved on the sides for the reception of the deltidial plate. 
Dorsal valve very depressed-convex, with a narrow and low mesial fold 
which is somewhat flattened at top : beak small but distinct, projecting 
but little beyond the line of the hinge. Area extremely narrow, and 
nearly rectangular to the area of the ventral valve. 
Surface marked by about twenty-four to twenty-eight fine simple rounded 
plications on each side, few of which reach to the beak. The plications 
are crossed by very fine concentric rugose striae, which are slightly 
arched upwards on the plications and on the mesial fold. 
Interior of the valves unknown. 
This species is usually readily distinguished by its extreme lateral extension, and 
by the elevated and more or less receding area, which is usually nearly as high or 
higher than the length of the ventral valve. There is frequently a slight incurvation 
at the apex of the ventral valve, and sometimes the area is nearly vertical. In this 
condition it approaches in general form to the young of S. medialis, which has 
always a proportionally longer ventral valve. 
The specimens of this species have usually a length of an inch or an inch and a quarter (as 
in the accompanying figure); while the largest individual observed is that figured on Plate 
xxxviil A, which has a width of more than two inches. • 
