2 
PALEONTOLOGY OF NEW YORK. 
the anterolateral margin with little variation. Beaks obtuse, rounded, 
anterior to the middle of the hinge. Umbo subtending an angle of 
about 130°. 
Ears large, triangular; posterior one the larger, and defined by the abrupt 
slope of the side of the umbo, while the anterior ear is separated by a 
distinct sulcus; lateral margins concave, becoming convex at the hinge¬ 
line. Byssal-sinus broad, rounded, well-defined and indicated on the ear 
by a sulcus extending to the extremity of the beak. 
The right valve is flatter and proportionally broader than the left. The 
limits of the ears are clearly indicated by the rapid slope of the umbo, and 
the absence of strong radiating lines of ornamentation. 
Test thin, marked by numerous regular alternating rays, which increase 
in number by interstitial additions, and become broader and stronger towards 
the margins. These radiating ribs are crossed by very fine, sharp striae of 
growth. On the ears the rays are nearly obsolete, and the lines of growth 
are sharper and stronger than on the body of the shell. 
The internal mould preserves, in a subdued degree, the ornaments of 
the exterior, and shows a simple, continuous pallial impression, extending 
nearly parallel to the margins, and terminating in a large sub-ovate muscular 
impression, which is posterior to the middle of the shell. The cavity 
beneath the beak extends over the ligamental area at that point, forming 
the cartilage-pit. In young individuals the ligamental area shows three or 
four small furrows at the beak, extending towards the extremities of the 
hinge-line. Three of these furrows measure 2 mm. across. In larger and 
older individuals, this furrowed ligamental area is much more marked, 
having frequently a width of 5 mm., and extends the entire length of the 
hinge, showing more numerous furrows. At the beaks the furrows are bent 
slightly outwards, otherwise, with gentle undulations, they run parallel to 
the hinge line. This feature is a constant character in this and several 
other species, and is similar to that in many forms of Pterinea and 
Lyriopecten. 
The dimensions of the shells of this species are very variable. Large 
