586 GEOLOGY OF OHIO. 



therefore a more elongate triangular appearance. Often the lateral bound- 

 aries of the median fold are rendered more distinct by broad, shallow 

 grooves, which begin 10 mm. or more from the beak, and extend anteriorly 

 along their border. Occasionally the anterior and lateral parts of the valves 

 are slightfy wrinkled radially, forming ver3' shallow, broad, indistinct radi- 

 ating plications. Very fine concentric striae of growth are readily detected 

 with a lense. 



The hinge area is quite variable in length. In the short-hinge forms 

 the area has a length equivalent to only half the width of the shell. In 

 the long-hinge forms the length of the area is equal to about four-fifths 

 of this width in the majority of shells, but occasionally longer hinge 

 areas are found, the longest being that of one specimen in which the 

 length of the area equaled six-sevenths of the width of the shell. 

 From this it may readily be seen that the general outline of the shell 

 varies from a quite triangular form, to one whose more nearly parallel 

 sides give it quite a quadratic appearance, excepting along its anterior 

 margin. In well preserved specimens the posterior part of the lateral 

 outline is alwa}^ more or less concave, making more prominent the 

 postero-lateral angles, especially in the shells with more quadratic outline. 



The hinge area of the pedicle valve is low, and outwardly inclined, 

 this inclination varying from an angle of thirty to one of seventy-five 

 degrees when measured with reference to the plane passing through 

 the lateral margins of the shell. The delthyrium is narrow, and is cov- 

 ered by a linear or moderately triangular convex plate, excepting at its 

 posterior extremity where a small circular ioramen is seen. In speci- 

 mens in which the outer surface of the shell at the beak has been worn, 

 this foramen is seen to be the mouth of a small tube which lies close to 

 the exterior of the shell, and this tube in good interiors is seen to open 

 by an equally small, round opening at the posterior extremity of the 

 muscular area, thus passing behind and under the callosity formed by 

 the thickening of the shell posteriorly. The straight anterior edges of 

 the hinge area are continued inwards, beyond this junction with the 

 teeth, but the} 7 do not join at the median line of the shell, leaving a 

 small notch anterior to the deltidium on either side of which there is at 

 times a slightly increased thickening of the posterior regions of the shell, 

 to support these inner angles of the hinge area. In one specimen the 

 shell was not thickened posteriorly beneath the hinge area, and here it 

 can be seen that the teeth are an outgrowth from the interior of the valve, 

 starting at a point on either side of the beak, under the hinge" area. 

 Thence they grow out in a very divergent direction, resting on the shell 

 beneath and being connected with, and supporting, the hinge area above. 

 As a rule the posterior parts of the shell below the hinge area are strongty 

 thickened, especially nearer the teeth. In that case the outer side of 

 the teeth is no longer so distinctly shown, but is merged into the callous 

 thickening of the shell laterally. The teeth project beyond the margin 



