PSEUDOSYRINX 407 



The dimensions of an incomplete but undistorted specimen are : length of 

 pedicle valve from beak to front 22 mm., length of brachial valve 16 mm., 

 width 38 mm., thickness approximately 22.5 mm., height of cardinal area 

 17 mm., width of sinus in front 11.5 mm., width of delthyrium at base 

 7.5 mm. 



Pedicle valve subpyramidal in form, the surface sloping from the umbo 

 to the lateral and anterior margins with a gently convex curve ; the 

 median sinus narrow and sharply defined at the beak, rounded in the 

 bottom, of moderate depth, becoming broader and less sharply defined 

 anteriorly; the beak small, pointed, slightly curved over the cardinal 

 area; cardinal area nearly flat except near the apex, where it is gently 

 arched just beneath the beak, the lateral margins not sharply defined but 

 rounding into the lateral slopes of the valve, sloping anteriorly from the 

 hinge-line at an angle of from 55 to 60 degrees to the plane of the valve, 

 the triangular, vertically marked, differentiated region on each side of 

 the delthyrium is rather narrow, the base of the triangle upon the hinge- 

 line being considerably less than one-half the total length of the hinge- 

 line; delthyrium narrowly triangular, the height nearly, or more than 

 twice, the width at the base ; each lateral slope marked by from 16 to 18 

 simple, depressed, radiating plications which originate along the cardinal 

 margin and grow successively smaller towards the cardinal extremities. 

 Internally, the dental lamellae are well-developed, and extend anteriorly 

 along the floor of the valve more than half way from the beak to the 

 front margin, their divergence being the same as that of the sides of the 

 median sinus; the muscular scars are narrow and not deeply impressed, 

 with a low longitudinal ridge dividing them along the median line ; the 

 delthyrial plate is only a little depressed below the level of the cardinal 

 area, and reaches less than half way from the apex of the delthyrium to 

 the hinge-line, its outer surface is nearly flat or slightly convex trans- 

 versely, and its free margin is a regularly concave curve. 



Brachial valve much shallower than the pedicle, subsemicircular in out- 

 line, with a rounded emargination in front opposite the mesial fold, the 

 surface curving more abruptly to the cardinal margin and becoming 

 somewhat compressed towards the cardinal extremities; mesial fold of 

 moderate height, rounded on top ; each lateral slope bearing simple pli- 

 cations similar to those of the opposite valve but much more strongly 

 defined. 



The minute surface markings of both valves consist of very fine elongate 

 papillae abruptly raised at the anterior extremity and merging into the 

 surface of the shell posteriorly, these papillae are arranged in concentric 

 rows and alternate with each other in position in succeeding rows. Shell 

 substance minutely punctate. Concentric lines of growth of varying 

 strength occur at intervals upon both valves and are sometimes crowded 

 toward the front margin. 



