588 GEOLOGY OF OHIO. 



anterior edge of the process where it curves strongly inwards is produced 

 somewhat anteriorly beyond its point of attachment so as to form a small 

 anterior projection. The bifurcated extremities of the process are 

 strongly grooved above in most specimens, the grooving however rarely 

 extends beyond the point of junction of these extremities, although 

 sometimes reaching the hinge margin as a much finer groove. In some 

 specimens the bifurcated extremities are evidently hollow interiorly, and 

 the grooves above named are then caused by the failure of the sides of 

 these tubular extremities to meet above. The crural plates are pointed, 

 rather triangular bodies, diverging strongly on either side of the car- 

 dinal process; their tips are often curved upwards; their triangular cross 

 section is caused by their rather acute upper margin from which the 

 flattened sides slope towards either side to the rounded lower surface of 

 the process. As a rule the teeth are quite prominent, but by the thick- 

 ening of the posterior parts of the shell, the teeth are at times almost 

 covered up, so as to leave only their tips projecting distinctly above this 

 callous thickening. The thickening takes place at first around the car- 

 dinal process and the crural plates. Thence it extends laterally to other 

 regions anterior to the hinge line. At first it is greatest near the process, 

 but in old specimens the thickening increases until it involves also regions 

 anterior to the cardinal process, and in the most pronounced case of 

 thickening, where the shell had a thickness of 8 mm., this maximum 

 measurement came from a region a short distance anterior to the cardinal 

 process. In these extreme cases the thickening causes a sort of very 

 broad convex transverse ridge in the interior of the shell, decreasing 

 rapidly posteriorly to the hinge line, and anteriorly to the muscular im- 

 pressions, to be described later. 



The pedicle valve is therefore moderately thickened near the hinge 

 line, towards either side ot the teeth, while the brachial valve is also thick- 

 ened near the hinge line, but the maximum is reached along the more 

 median portions of the shell r near the teeth and the cardinal process, or 

 just anterior to the same ; the strongly concave inner surface of the beak 

 of the brachial valve is usually filled up in this way. It may be 

 imagined from the descriptions above given that posterior portions of 

 brachial valves, with their teeth and cardinal process, which are quite 

 commonly found as fragments separated from the remainder of the shell, 

 may present quite a series of variations, greater perhaps than one is 

 accustomed to expect in shells of the same species. 



Except over the thickened area already described the pedicle valve is 

 very thin. The inner edges of the thickened portion of the shell continue 

 for a short distance the border of the muscular area furnished by the 

 base of the teeth, but this thickening soon disappears anteriorly, so that 

 the muscular area is not defined anteriorly. In one specimen the trian- 

 gular space thus partially bordered seemed indistinctly striated radiately. 

 Towards the anterior of this striated area a sort of very broad median 



