532 MEMOIRS OF THE CARNEGIE MUSEUM. 



higher and much more rounded than the present species ; the latter is more elongated 

 and more ])ointed behind, and has, besides, a smooth epidermis {Cf. Von Ihering, 

 1890, p. 163). Since it is imi)ossible for me to find any other South American 

 species of Diplodoti, the description of which answers to the present species, we 

 must regard the latter as new. 



Description of Shell. — Shell small to medium (maximum length 53 mm.), 

 rather thin. Outline subovate or subtrapezoidal, distinctly oblique, higher behind, 

 narrowed anteriorly, the obliquity being most pronounced in older specimens. 

 Height 56 to 75 i^r. ct. of length (against 54 pr. ct. in ellipticus, and 86 or 87 pr. ct. 

 in rotundus). Valves not gaping. Dorsal margin straight or gently convex, form- 

 ing a rather distinct or obtuse angle with the posterior margin, which descends ob- 

 liquelj^ and rather steepl.y, is nearly straight or gently convex, and curves into 

 the lower margin without forming a distinct posterior point. The posterior ex- 

 tremity of the shell is located relatively low and is only moderately elevated above 

 the base-line. Lower margin with its lowest point located rather posteriorly, 

 vertically below the posterior end of the ligament, or even behind it, strongly as- 

 cending in a curve toward the posterior end of the shell, Init nearly straight or 

 veiy slightly convex in its anterior part, and sloping upward toward the anterior 

 margin, so that the anterior portion of the shell is distinctly narrower than the 

 posterior, producing thus the oblique appearance of the whole shell. 



Valves comparatively compressed. Diameter 28 to 45 pr. ct. of length. 

 Greatest convexity and greatest diameter situated well back upon the posterior 

 ridge, which, however, is very indistinct and broad. Sides of the shell in front of 

 posterior ridge not very convex and rather flattened. Posterior slope compressed, 

 sometimes with a faint trace of a radial groove, somewhat elevated (wing-like) 

 toward the upper posterior angle. Beaks not swollen, and hardly elevated above 

 the hinge-line, located at from 22 to 28 pr. ct. of the length. Beak-sculpture 

 sharp and fine, distinct, but restricted to the region near the beaks. There are 

 fifteen 'to eighteen radial bars, the ninth and tenth in the middle, joined and v- 

 shaped at their lower ends. The longest bars (6 to 8 mm.) stand upon the posterior 

 ridge, the anterior ones being distinctly shorter. There are a few anterior bars 

 and upon the posterior slope a few posterior bars, and sometimes traces of oblique 

 wrinkles. None of the bars are distinctly granular, and a comparatively slight 

 degree of erosion obliterates all traces of beak-sculpture. Lunula indistinct and 

 narrow, visible only in older shells. 



Epidermis not shining, but rather rough. This is due to a great number of 

 fine, irregular, concentric lines, which, when well-preserved, are lamellar and 



