94 PALAEONTOLOGY OF NEW YORK. 



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

 metre. Left valve marked with from six to ten or twelve strong rounded 

 iavs, which originate at or near the beak and continue simple to the margin. 

 The interspaces arc marked by smaller, alternating costae, 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 backwards over the stronger radii. The cardinal 

 expansions are marked with rays; those on the ear in well-preserved speci- 

 mens, are stronger than on the wing. 



In the partially exfoliated condition, and in the casts, the ears show the 

 concentric stria 1 : and the wings show evidences of the rays. In some con- 

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

 there are undulations of growth which interrupt the rays and increase their 

 nodose aspect. 



In the right valve the surface is marked with 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 casts of the interior sometimes show traces of the stronger radii ; but 

 usually they are not preserved. 



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

 gradation from the finer radii to the stronger ones, and some specimens are 

 quite destitute of 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 arcuation of the valves, the prominence of the beaks, 

 and the thickness of the test. In rare examples, the stronger rays' below the 

 umbo are broken up and merged with the finer striae. 



The pallia! line originates in a small, deep, anterior muscular impression 

 just beneath the anterior tooth, and extends nearly parallel with the margin 

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

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

 muscular impression directly below the posterior extremities of the lateral 

 teeth. 



