284 NEW YORK STATE MUSEUM 



The greater number of these lineate specimens are sculpture casts and 

 do not of themselves clearly demonstrate the external character of this 

 radiation, but on some specimens from the limestone the shell is retained 

 and indicate that the striation is strictly exterior and ornamental. 



Description. Shell of considerable size, typically suborbicular, with 

 central beak which is incurved and minute and oblique. The apical part of 

 the beak is sometimes set off by a low transverse thickening which thus 

 brings the embryonal shell into prominence. 



Hinge line short and straight; cardinal area narrow, elongate, trian- 

 gular, not elevated. 



Surface quite evenly convex from the umbo outward ; often abruptly 

 folded downward at the periphery. The ornamentation normally consists 

 of sharp, imbricating concentric striae which may form successive festoons 

 or undulations far apart, covered by lines close together, or the lines may 

 be all distant or all approximate. These more distant lines are sometimes 

 limited to the body of the valve, while about the margins they are crowded, 

 or again they are to be found only about the margin. Sometimes internal 

 casts show a concentric undulation which manifests itself on the surface 

 only in a fasciculation of the striae. 



There may be no radial striae whatever on the shell, but in the major- 

 ity of instances some evidence of them is to be seen. Sometimes, though 

 seldom, there appear three or more very fine lines on each side of the beak 

 close to the cardinal area. The lines may also cover the anterior and pos- 

 terior slopes of the shell, or extend over the entire surface, cancelating the 

 finer concentric lines and minutely crenulating the margin. All these lines 

 are exceedingly fine, but become more prominent in the old shells near the 

 margin. 



These shells are subject to variations in some other respects. The 

 contour of the surface is not uncommonly modified by the abrupt concentric 

 deflection of the peripheral region, and, if this has occurred more than once 

 in the .life of the shell, a very uneven surface may result. Again, while 

 shells which agree with the original in having a central beak are sufficiently 



