580 GEPLOGY OF OHIO. 



thickening of the shell suddenly becomes very slight. Anterior to 

 this line lie the anterior adductor scars, in an area where the shell is but 

 slightly thickened; their anterior margin is but rarely well defined, and, 

 when best indicated, consists of a few striations there where even the 

 meagre thickening over this area totally ceases. The posterior adductor 

 scars are separated by a low fiattish elevation, often provided with a me- 

 dian groove. The scars on either side are traversed by longitudinal ridges, 

 parallel to the lateral boundaries of the scars, with reference to which they 

 may be regarded as strise of growth. Crossing these and sometimes al- 

 most concealing them are curved strise following the inner contour of the 

 anterior boundary of the adductor scars, with reference to which they 

 may be regarded as strise of growth. In one specimen a single striation 

 parallel to the outer contour of the anterior boundary of the scar is seen, 

 and this is also a striation of growth. A much narrower low ridge 

 branches from the posterior end of the -median elevation and extends a 

 little beyond the middle of each scar where it usually becomes indistinct. 

 A median ridge of moderate width, decreasing in size anteriorly, separates 

 the anterior adductor scars. 



The muscular area of the pedicle valve is sharply defined laterally 

 by a limiting ridge which is *low anteriorly, where the thickening of the 

 valve abruptly diminishes but which posteriorly becomes developed into 

 thin very acute erect plates, serving as a support to the teeth, this pos- 

 terior portion being rarely well preserved. Along the bottom of this 

 muscular area extend several diverging striations, on a moderately eleva- 

 ted flattened surface. In the specimens where this part of the shell had 

 undergone but very little thickening three striations were seen, corre- 

 sponding to three grooves between two plications in the sinus, as seen from 

 the exterior of the valve. In cases where the thickening of the shell . 

 over this area is more pronounced, the strise are sometimes quite well 

 preserved, but often in less distinct form: at other times however the 

 central striation disappears more or less, and the other two strise seem 

 to be less divergent than they were during their earlier growth. The 

 linear area between these strise coresponds to the adductor muscles.; the 

 elongate areas on either side, to the diductor muscled; no line of dilimita- 

 tion serves to fix the boundary of the pedicle muscle in the specimens at 

 hand, but striations following the anterior outline of the area mark the 

 inner surface of the scar in the more thickened specimens. Ovarian im- 

 pressions characterized by the presence of numerous short subparallel 

 strise, following in a general way the direction of the plications of the 

 exterior, shell, are often seen covering a considerable area on either side of 

 the muscular area in the pedicle valve, but in the opposite valve these 

 strise, if present, are confined to a very narrow area on either side of the 

 muscular scars. Vascular markings evidently extend from the anterior end 

 of the muscular scars of the pedicle valve forwards but their course could 



