142 



Pteronitella venusta. (N. sp.) 



Plate 9, figs. 5, 5«. 



Description. — Hinge lino straiglit, extending a short distance in front of 

 the beak, and, behind the umbones, backwards to about the posterior extre- 

 mity of the body of the shell. Anterior extremity, in the upper fourth part 

 of the height, forming a small triangular projection in front of the beaks ; 

 the lower three-fourths sloping downwards and gradually curving back- 

 w^ardSj into the ventral margin. The latter is uniformly rounded, the 

 greatest depth being about the mid-length or a little behind. The most 

 projecting point of the posterior extremity of the body of the shell is a 

 little below the mid-height. Below this point the outline curves down- 

 wards and forwards, passing gradually into the ventral margin. Above 

 the same point there is a curve, at first forwards, and then backwards, to 

 the posterior extremity of the hinge line, the sinus thus formed being at 

 about three-fourths the height of the shell. 



The right valve is nearly flat ; the left is moderately and obliquely 

 convex from the umbones to the lower half of the posterior extremity. 

 The posterior wing is either flat, gently concave, or very slightly convex. 

 The anterior wing is flat for a small space at its extremity ; behind this, 

 shghtly convex, and then with an obscure depression running from the 

 beak, downwards and sloping slightly backwards, for one-half the height 

 of the shell or a little more. The beak of the right valve is situated at 

 about the anterior fifth . 



The best preserved impression of the hinge-line shows four anterior 

 teeth. The first of these slopes downwards and forwards ; the other three 

 downwards and backwards. There appears to be two lateral teeth in the 

 right valve and one in the left. They are about one-fourth the length of 

 the shell, and situated in the middle part, close to the hinge-fine. 



Surface concentrically striated. On the anterior wino; the striae are 

 lamellose, and closely crowded together. On the body of the shell (of 

 the large specimen figui'es, fig. 5.) the striae are for the greater part fine, 

 simple, sharply elevated ridges, about one line distant from each other, 

 and with nearly flat spaces between. The surface of the smaller speci- 

 mens is more closely striated. On the posterior half of the shell, the 

 striae, above the mid-height, curve forwards, and then backwards and up- 

 wards to the hinge-line, thus forming a rounded sinus w^ith the convexity 

 towards the umbones. 



Length of the largest specimen collected, thirty-two lines, on the 

 hinge line. Height, at a little behind the middle, sixteen lines. Length 



