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 
rays, which originate at or near the beak and continue simple to the margin. 
The interspaces are 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 striae ; and the wings show evidences of the rays. In some con¬ 
ditions the rays are nodose from the concentric laminae, 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 pallial 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. 
