LEBT^ENA 51 



vascular impressions originates at the anterior border of the muscular 

 scar in the middle of the valve, it passes towards the anterior margin a 

 short distance and divides into two lateral limbs which have a direction 

 subparallel with the anterior and lateral margins of the valve, each lateral 

 limb giving off branches at nearly right angles towards the margin of the 

 valve, each branch divides into two or three before reaching the margin, 

 and those in the anterior portion of the valve often anastomosing ; between 

 the two lateral limbs of the vascular markings and the muscular scars 

 the inner surface of the valve is thickly papillose, with a few similar, but 

 more scattered papillae within the vascularly marked area. 



Brachial valve slightly concave near the beak, becoming flattened or 

 slightly convex to the line of geniculation of the pedicle valve, at which 

 line the valve is abruptly deflected to conform with that valve ; mesial 

 fold entirely obsolete or represented by a slight, broad, low, ill-defined, 

 longitudinal elevation across the deflected border of the valve ; cardinal 

 area narrower than that of the opposite valve and meeting it along the 

 hinge-line in an angle of 45 degrees or less, the posterior extremity of the 

 cardinal process protuberant at the middle of the cardinal area and sur- 

 rounded by the chilidium. Internally a conspicuous, elevated, angular 

 ridge follows the external line of deflection of the valve except near the 

 cardinal margin, where its terminal portions curve towards the center of 

 the valve; the ridge increases in prominence with age, and gives to that 

 subelliptical portion of the valve bounded by it, a deeply concave contour ; 

 the external surface of the ridge slopes steeply to the margin of the valve 

 except towards the cardinal extremities where it is deflected laterally; 

 the cardinal process bifid; the muscular scars occupy a subrhomboidal 

 area bordered by an elevated ridge and are divided by a median ridge ; 

 the vascular markings have an arrangement similar to those of the oppo- 

 site valve ; the surface sloping from the marginal ridge to the outer mar- 

 gin of the valve is marked by rather coarse, elongate papillas irregularly 

 arranged in radiating lines. 



The surface markings of both valves consist of a series of conspicuous, 

 concentric corrugations covering the flattened portions of the valves; 

 these corrugations are variable in size upon the same individual, growing 

 successively coarser from the beak to the margin, and also vary among 

 different individuals, the minimum width being less than 1 mm. and the 

 maximum 2 mm. or more ; these wrinkles are often more or less discontin- 

 uous and sometimes divide or anastomose, and towards the cardinal ex- 

 tremities their direction is usually gradually deflected laterally ; crossing 

 the concentric wrinkles and continuing to the shell margin, are a series of 

 fine, rounded, radiating costse which increase by bifurcation and intercal- 

 ation, about three or four of these usually occupying the space of one 

 millimeter ; the entire surface also marked by exceedingly fine concentric 

 lines of growth. 



