•266 PALEONTOLOGY OF JJEW-YOUK. 



Cyitiiia hiplicata. 



PLATE XXVII. 

 fjrlia biplitala .• Hall, Tenth Report on the State Cabinet, p. 1C5. 1857. 



Shell scmipyramidal (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. 

 Vjjntkal 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 is 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 striae, 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 



