338 PALAEONTOLOGY OF NEW YORK. 



more or le.ss divergent marks of the impressions of the lateral sliding muscles. 

 The middle portion of the thickened margin slopes to the center of the shell, 

 forming in the larger valve a line concave as to the beak, and in the smaller 

 valve a sinus. The lateral portions of the thickened area merge gradually into 

 the thin margins of the valves. Beneath the hinge-line in both valves is a 

 narrow median septum, fainter in the small valve, and in both a scarcely visible 

 ridge discernible only in oblique light. On eitlier side of the septum begins a 

 furrow which increases in depth and passes through the thickened area in a line 

 parallel to the margins of the shell. 



" The impressions of the vascular trunks of the mantle are continued from 

 these furrows into the anterior portions of the valves and parallel to their 

 margins. Secondary vascular sinuses radiate in great numbers from the main 

 trunks toward the margins and center of the shell. 



" In the median line of the larger valve, between the median septum and the 

 anterior margin of the thickened cardinal region, lies a deep cordate pit, having 

 its apex directed toward the anterior margin of the shell and with a shallow 

 median furrow. In the sinus of the smaller valve is a faint median swelling, 

 extending beyond the center of the shell and divided longitudinally by an 

 obscure groove. 



" Muscular imjjressions in five pairs for each valve; two of the adductors and 

 three of the sliding nmscles. The impressions of the adductors lying difectly 

 beneath the cardinal margin, are separated in the larger valve and in juxtapo- 

 sition in the smaller. The anterior pair in the larger valve lies on either side 

 of the cordate pit, in the smaller at the apex of the corneous callosity, extending 

 into the middle of the valve and forming the sinus. Of the sliding muscles 

 there is a pair on each side of both valves, close to the cardinal margin and 

 between the lateral furrows and the margins of the shell. In the larger valve 

 these scars are in juxtaposition, but in the smaller they are further apart, 

 touching only at their extremities. The third pair of these impressions in 

 the larger valve lies close against, but outside the anterior adductors ; in 

 the smaller valve on either side of the anterior extremity of the median 

 swelling, in the sinus, and nearer together than the scars of the anterior ad- 

 ductors. The scars of the paries passes closely about the muscular impres- 

 sions, crossing both lateral furrows and terminating in the central portion of 

 the cardinal mar";in." 



