FOSSILS OF THE CLINTON GROUP. 587 



i 



of the hinge area, at a broad angle, forming a deep notch laterally, ren- 

 dered still more distinct by a narrow groove outlining the anterior edge 

 of the hinge area. The inner surface of the teeth is flat, the anterior 

 margin being acute, and curved upwards, the tip of the teeth being acute 

 and rather pointed, and rising a little above the plane of the hinge area. 

 The brachial valve does not possess a real hinge area. The posterior border 

 of the valve is an almost straight, sharp ridge, from which diverges at a 

 very low angle another fine, sharp, almost paralled ridge, starting near 

 the crural plate and extending almost or quite to the postero-lateral 

 angles of the shell. The narrow groove between these fine' ridges or 

 striae serves as a resting place for the acute anterior outline of the hinge 

 area of the pedicle valve. In one specimen this margin of the pedicle 

 valve seems to have slipped out of the groove at one time, and a new 

 pair of striae, at a slightly greater angle was formed anteriorly, provid- 

 ing a new groove as a socket for the anterior margin of the hinge area 

 of the opposite valve. The space between all these striae belongs to the 

 hinge area. This may be all of the hinge area of the brachial valve 

 which is developed, but not infrequently anterior to these striae, a long 

 and very narrow surface, with a straight anterior margin is seen, more or 

 less flattish towards the middle regions of the) hinge line, and strongly 

 inclined outward. Posterior to the teeth it usually shows fine parallel 

 striae of growth. This is the remainder of the hinge area.* In many of 

 these specimens it may be seen that the delthyrium is closed, since the 

 anterior 'margin of the hinge area turns up abruptly over the outer pos- 

 terior side of the cardinal process, forming there a callosity and leaving 

 behind it a groove into which fits the indentation of the hinge area of 

 the pedicle valve, the anterior edge of the deltidium. In some valves 

 the co'ntinuous connection of this callosity with'the anterior margin of 

 the hinge area is not seen, so that were it not for the other specimens it 

 would seem to be only the posterior thickening of the cardinal process itself. 

 The cardinal process varies considerably in its minor features, but 

 it is always very large, and deeply bifurcated ; the supporting region of 

 the beak is always strongly incurved, so much in fact that the cardinal 

 process after growing downward and inward for a short distance, curves 

 abruptly backward and occupies, in shells with a short hinge area, and more 

 convex pedicle valves, an almost horizontal position, whereas in shells 

 with larger hinge area, and flatter pedicle valves the process is obliged 

 to take a more inclined position, its extremities being apparently directed 

 more towards the anterior of the opposite shell, in order that the card- 

 inal process may find room within the pedicle valve. The undivided portion 

 of the process may be more, or less rounded or flattened laterally, often so 

 as to give its inner median line quite an acute edge, especially anteriorly ( 

 or it may be flattened above and below so as to give the lateral edges a 

 more acute outline, or the broadening of the cardinal process may be of such 

 a character as to give a general triangular cross-section. At times the 



