582 GEOLOGY OF OHIO. 



incurved at the top. In both valves the hinge area is considerably nar- 

 rower, than the width of the shell. The anterior margin in both is a 

 straight line, parallel to which there are often striae of growth, which are 

 . best seen in the hinge area of the pedicle valve. The delthyrium in the 

 pedicle valve is open, that of the brachial valve is almost if not quite 

 closed by the cardinal process. This, brachial process varies considerably 

 in different specimens. In form it resembles usually a triangular plug 

 almost filling up the delthyrium, excepting along a narrow groove on 

 either side, which serves to define the neighboring boundary of the hinge 

 area. It juts out a considerable distance beyond the hinge area at times. 

 In one young specimen the process does not show any definite lobation, 

 in another it is marked above by three, strong, acute lobes or ridges. In 

 an older specimen the cardinal process is triangular, and flattish above, 

 divided by a not deep, narrow, median groove. In other specimens only 

 the anterior edge of the process is notched by a groove which extends for- 

 ward for some distance on the anterior prolongation of the cardinal pro- 

 cess which in turn is continuous with the ridge separating the muscular 

 scars. According as the anterior margin of the process is indefinite or a 

 more or less straight line, the appearance of the process will vary greatly. 

 Even in old specimens the process is at times undivided, and prominent; 

 or bilobate, owing to a single median furrow; or it is trilobate, owing to three 

 diverging more or less prominent and often acute ridges on its upper 

 side 1 ; or, lastly, quadrilobate, owing to a slight median elevation of the 

 process anteriorly, which is not only itself divided by a median furrow, 

 but which is also bordered on either side by a groove, dividing the 

 upper surface of the process into four ridges. The inner margin of the 

 hinge area along the delthyrium may or nlay not be slightly raised into 

 a fine bordering ridge, but in either case the sharp crest of the crural 

 plates can be, seen to be the continuation of this margin of the hinge area. 

 These crural plates are flattened laterally, and rest for support on thick- 

 ened callosities, developed in the posterior part of the shell, near the 

 beak. The crural plates jut out strongly from the inner surface of the 

 shell; their elevation above the plane of the hinge area is however much 

 less than might at first be imagined, the hinge area of the brachial valve 

 having a quite strong outward inclination. A quite strong notch or 

 rounded impression in the outer angle between the plate and the hinge 

 area forms the dental socket, and together with the supporting callosities 

 adds to the rather heavy appearance of the crural plates of this species, 

 In the pedicle valve the teeth are also evidently the continuation of the 

 inner margin of the hinge area, along the delthyrium. The teeth, however, 

 usually bend a little towards the ' lateral side of the shell and at the 

 same time are moderately elevated above the plane of the hinge area, es- 

 pecially along their inner acute outline. They are supported by callosities, 

 which are plate-like anteriorly, and which serve to limit the muscular 

 impressions of this valve posteriorly. 



