31 



Strophomena puNCTULiPERA, (Hall.) Geol. Rep., 4th Dist. 

 Strophodonta punctdlifera, (Hall.) Pal., N. Y., vol. Ill, p. 188, pi. 21. 23. 



Description. — Shell semi-elliptical or sub-triangular; hinge-line usually 

 a little greater in length than the width of the shell ; sides in the upper 

 half more or less concave, sometimes straight ; anterior margin broadly 

 rounded, occasionally sub-angular in the middle ; length on the hinge- 

 line from one to three inches, usually about two inches. Length from 

 one-sixth to one-third less than the width. 



Ventral valve concave, the umbo and a small space around it convex ; 

 sometimes a shghtly elevated mesial fold runs from the umbo to the front 

 margin ; beak, small, incurved down to the cardinal edge, and often pro- 

 jecting a little over it ; dorsal valve convex, with a portion in the middle 

 of the upper half more or less concave. In some specimens a shallow 

 mesial depression extends to the front. The valves are either shghtly 

 curved or nearly, flat, or more or less abruptly geniculated at a variable 

 distance from the hinge-line. 



Area of ventral valve about two Hues high, at the foramen, in a speci- 

 men of average size ; slightly incurved beneath the beak, and for a short 

 distance on each side. It varies in the amount of its inchnation, sometimes 

 forming nearly a right angle to the marginal plane, but usually sloping 

 backwards, more or less. In a specimen twenty-four lines wide on the 

 hinge-lines, the edge of the area is denticulated for the length of five hues 

 on each side of the foramen ; the remainder out to the cardinal angles, 

 smooth and grooved,^as in S. euglypha and others of this group. The 

 foramen is closed by a convex deltidium, two lines wide at its base, and 

 the area striated for about five lines on each side. The area of the dorsal 

 valve is narrow, less than one hne wide, and forms an angle of 90^, or 

 somewhat more, with that of the ventral. 



Externally the shell is covered with small angular ribs, from four to 

 six in the width of two lines, increasing both by intercalation and bifurca- 

 tion ; often they become finer towards the margin than they are in the 

 middle portion of the upper half. In some specimens, where the surface 

 is perfect, there are no indications of either punctures or tubercles, on the 

 ribs or in the grooves between ; but, when the shell is exfoliated, the 

 grooves are seen to be distinctly punctured or pitted. The inner sur- 

 face of the shell is covered with small tubercles, which are larger and 

 more dansely crowded together in the upper half, on each side of the 

 muscular impressions, than they are elsewhere. 



The muscular impressions are not well exhibited in any of our specimens. 

 The usual size of this species, is from twenty to twenty-eight lines wide 

 •on the hinge-line. It varies, however, both in the size and in the amount 



