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 stria 1 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 pallia! 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 Pterixea and 



I,\ Klol'l'.CTEN. 



The dimensions of the shells of this species are very variable. Large 



