154 PALEONTOLOGY OF NEW YORK. 



Tlic'se striae become more conspicuous upon tlic maruiiis iiiul upon the wings, 

 wlicrr tlu'V ;iri' ol'tcn liiiililv lamcllose. The distant lamellae wliicli mark 

 the lioilv of the shell are sometimes continued upon tlic liinge, and becoming 

 stronger, curve over tlic margin, giving it a sharply annulated aspect, 

 which, when preserved in the impression of the ]iinge-l)order, might be 

 mistaken for a more important organic marking. Partially exfoliated shells 

 show some obscure radiating lines which belong to the intimate shell-structure, 

 and are not external. (These strise are too conspicuously represented in 

 figure 6 of plate xii.) 



Posterior muscular impression situated near the middle of the posterior 

 slope, and marked by irregular vascular striae ; from the lower anterior side of 

 this impression the pallial line extends slightly downward, curving forward, 

 and continuing in a line nearly parallel to the margin it terminates in a 

 small muscular impression within the rostral cavity. The pallial line is 

 frequently pitted or interrupted in its course, and from these ])its ladiating 

 grooves extend toward the margin of the shell. 



Ligamental area extending the entire length of the hinge, comparatively 

 wide, grooved longitudinally with fine, continuous parallel lines. Hinge with 

 one or two linear, sliglith' oldique, lateral teeth on the jwsterior side, and 

 with numerous erect or oblique irregular folds along the cardinal line below 

 the ligamental area, which terminate on the inner margin in distinct crenu- 

 lations. The cardinal wrinkles are smaller and more regular anteriorly, 

 becoming large and irregular posterior to the beak. 



Three large specimens have the following respective dimensions: length 

 'JO, 70, and 81 mm., height 100, 99, and 81 mm. ; hinge-line 100, 90, and 90 

 nun. An example of medium size has a length of 55 mm., height 76 mm., 

 and hinge-line 48 mm. A large, erect individual measures 72 mm. in length, 

 and 90 mm. in height. A small right valve has a length of 23 mm., height 

 27 mm., and hinge-line 33 mm. 



This is an abundant species and attains a very large size in older individuals. 

 In its different conditions of growth and preservation, it presents a very great 

 variety of aspect, and it becomes extremely difficult to determine the limits 



