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PALAEONTOLOGY OF NEW-YORK. 
CrEKUS &PIRIFERA (Sowerby). 
The Genus Spirifera, established by Sowerby in 1815, embraces a very 
natural group of shells, which are for the most part very readily distin¬ 
guished by their external form and characters. The presence of internal 
spires is not alone sufficient to distinguish them, since these organs are 
common to other very distinct genera. 
The prevailing and characteristic form of the shells of this genus is 
somewhat trigonal; but they vary to subelliptical, ovoid and subcircular 
in outline. The hinge-line may be shorter or longer than the greatest 
width of the shell, and the cardinal extremities are sometimes obtusely 
rounded, and often produced into wing-shaped extensions which termi¬ 
nate in acute points. The surface is smooth, or marked by radiating 
costae, presenting a great variety of aspect and ornament; and the centre 
of the valves is usually marked by an elevated fold on the one valve, 
with a corresponding sinus in the other : this feature, however, is 
more or less obsolete in some of the species. The shell-structure is 
fibrous. 
The valves articulate by means of teeth and sockets. The beak of the 
ventral or larger valve is more or less elevated above the other, and may 
be straight or recurved. The area or space beneath the beak may be nar¬ 
row or large, flat or concave, or inclined forwards. This area is divi; 
ded by a triangular fissure, which is often more or less closed in its 
upper part by a pseudo-deltidium. In the dorsal valve the area is narrow, 
usually linear, with a wide fissure, which is partially filled by the car¬ 
dinal process. 
In the ventral valve the triangular fissure is bounded by vertical 
shelly plates, which extend from the beak to the bottom of the valve, 
and support, on each side at the base of the fissure, a short and usually 
strong hinge-tooth. These fissure-walls are usually short diverging plates; 
but they are sometimes nearly parallel throughout their length, and reach 
to the middle of the valve. They sometimes converge so as nearly to 
