!U PALAEONTOLOGY OF NEW YORK. 



Test strong, iincrcims, often having a thickness of more than one milli- 

 iiK'tre. Left valve marked with from six to ten or twelve strong rounded 

 rays, which originate .at or near the l)eak and continue simple to the margin. 

 Tlie interspaces are marked bv smaller, alternating costtB, increasing by 

 interstitial addition as the shell increases in size. The surface is also orna- 

 mented with strong, elevated, concentric lamellose striae of growth, which are 

 more or less distinctly bent liaekwards over the stronger radii. The cardinal 

 expansions an- niarkeil witli rays; ihosv on tlic rai- in well-preserved speci- 

 iiiciis, are .stronger than on the wing. 



Ill the ])artially exfoliated condition, and in the casts, the ears show the 

 concentric stria? ; and the wings show evidences of the rays. In some con- 

 ditions the rays are nodose from the concentric laminge, and in older examples 

 there are undulations of gi-owtli which interrupt the rays and increase their 

 nodose aspect. 



In the right valve tlie surface is marked witli concentric lines which are 

 more crowded on the cardinal expansions, and with a few strong radii on the 

 wing, which are sometimes quite obsolete. 



The I'asts of the interior sometimes show traces of the stronger radii : luit 

 usually they are not preserved. 



ill the various phases of maceration and exfoliation, the specimens present 

 gradation from tlic liner railii to the stronger ones, and some specimens are 

 quite destitute ol surface markings. They vary also in the number of 

 stronger radii, the development of the wing, the comparative length and 

 width of the body, the arcuatiou of the valves, the prominence of the beaks, 

 and the thickness of the test. In rare examples, the stronger ray.s below the 

 umbo are broken n|i and merged with the finer striae. 



Tlie jiallial line originates in a small, deep, anterior muscular impression 

 just beneath the anterior tooth, and extends nearly })aiallel with the margin 

 of the valve almost two-thirds the tlistance from the beak to the base of the 

 shell, where it is aliruptly recurved, terminating in a large, ovate, posterior 

 muscular impression directly below the posterior extremities of the lateral 

 teeth. 



