286 Thirty-fifth Report on the State Museum. 



hinge, curving forward over the hinge margin. In rare instances 

 the beak does not extend quite as far as the margin of the hinge. 

 Umbo of the left valve prominent, gibbous, limited by the sulci of 

 the cardinal expansions, and subtending an angle of from 30° to 

 60°. Beak of the right valve depressed, rounded, not rising above 

 the hinge; umbonal region depressed-convex, gradually merging 

 into the body of the valve, which is concave below. Wing large, 

 triangular, nearly flat, varying in proportions, defined (in the left 

 valve) by the post-cardinal slope; margin concave; extremity acute- 

 rounded. Ear of left valve a simple rounded convex lobe or auricu- 

 lation, defined by a distinct, broad, rounded sulcus ; margin rounded 

 above, sinuate at its junction with the valve, forming the byssal 

 sinus. The cardinal expansions of the right valve are similar in 

 dimensions to those of the opposite valve ; nearly in the same plane 

 with the body of the shell, and defined only by the surface striae 

 and the sinus. 



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

 millimetre. Left valve marked with from six to ten or twelve 

 strong rounded rays, which originate at or near the beak and con- 

 tinue 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 ornamented with strong, ele- 

 vated, concentric lamellose striae of growth, which are more or less 

 distinctly bent backward over the stronger radii. The cardinal ex- 

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

 specimens are stronger than on the wing. In the partially exfoli- 

 ated condition, and in the casts, the ears show the concentric striae; 

 and the wings show evidences of the rays. In some conditions the 

 rays are nodose from the concentric laminae, and in older examples 

 there are undulations of growth which interrupt the rays and in- 

 crease their nodose aspect. In the right valve the surface is marked . 

 with concentric lines which are more crowded on the cardinal ex- 

 pansions, and with a few strong radii on the wing, which are some- 

 times 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 



