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PALAEONTOLOGY OF NEW-YORK. 
Cyrtina Mplicata. 
PLATE XXVII. 
Cyrtia biplicata Hall, Tenth Report on the State Cabinet, p. 165. 1857. 
Shell semipyramidal (semielliptical in the dorsal view); length and 
breadth about as two to three, with a height of area less than the length 
of the shell : hinge-line equal to the greatest width of the shell. 
Yentral valve extremely elevated, semipyramidal, curving abruptly to 
the front and sides : the length from the apex to the cardinal extremity 
is often about the same as from the apex to the front of the shell. 
Mesial sinus somewhat deep, subangular in the bottom, and defined at 
the margins by a strong fold or plication : apex elevated, sometimes 
attenuate and slightly incurved over the high area, which^s either flat 
or slightly arcuate ; fissure narrow and partially or entirely closed by 
a convex pseudo-deltidium. 
Dorsal valve depressed-convex, and sometimes nearly flat towards the 
cardinal extremities : mesial fold moderately elevated, and strongly 
defined by a furrow on each side ; area extremely narrow. 
Surface marked by numerous fine radiating strias, and sometimes by 
distinct and even strongly marked imbricating lines of growth. The 
entire shell-structure is punctate. 
Where the shell is preserved, the mesial sinus is bounded on each side 
by a distinct elevated fold having a depression on the outside of it, and 
this feature is often shown in the casts. The median septum extends for 
one-third or more of the length of the ventral valve, and its extension 
is shown in the fissure beneath the pseudo-deltidium. From the little 
that can be seen of the spire, in specimens examined, it is arranged as 
in Spirifera. In this example the spire may be displaced so as to appear 
in a transverse position. In two species where the position of the spires 
are clearly determinable, they are directed towards the apex of the ven¬ 
tral valve; but in these species the transverse extension is less than the 
elevation of the valve, and I can conceive it probable, that in species of 
the genus having a low area, with considerable lateral extension of the 
