FOSSILS OF THE CLINTON GROUP. 589 



elevation appears, which may have something to do with the strong in- 

 dentation of the anterior border of the muscular area. The anterolat- 

 eral edge of the area was probably rounded, but our specimens only 

 allowed a faint suggestion of such a rounding in one case. At the poste- 

 rior end of the muscular area, w T ithin the anterior end of the teeth, the 

 anterior edge of the callous posterior thickening is often slightly and 

 rather roundly indented. This is the resting place for the distal extrem- 

 ities of the bifurcated cardinal process. The broad oblique flange-like 

 teeth of the pedicle valve rest firmly behind the acutely pointed rather up- 

 turned crural processes. From a point near the hinge area and a little to 

 one side of the teeth of the pedicle valve, two or three narrow parallel 

 grooves extend antero-laterally across that portion of the shell where 

 usually ovarian markings would be looked for. These grooves however 

 seem to be vascular markings. 



In the brachial valve at least sufficient thickening occurs anterior to 

 the great posterior thickening of the shell, already described, to render 

 more or less distinct the muscular impressions of this valve. These in 

 mature specimens are usually quite distinct. The muscular area is di- 

 vided by a median ridge, which may be low and indistinct posteriorly in 

 shells in which this region does not seem to have been greatly thickened. 

 In proportion however as this posterior thickening increases, the posterior 

 end of this median ridge becomes more prominent by the development of 

 two lateral furrows, which terminate abruptly at their deeper posterior 

 end, and thus give a high relief to this part of the median ridge. These 

 grooves have their posterior termination at the anterior edge of the region 

 of marked thickening, and therefore are quite a distance anterior to the 

 crural plates and cardinal process. Anteriorly these defining grooves 

 rapidly decrease in depth; the median ridge, rounded and strong poste- 

 riorly, becomes less elevated, broader and flatter anteriorly ; a median 

 groove begins to appear usually before the anterior adductor muscular 

 impressions are reached, and increases in distinctness between the lat- 

 ter. Between the more anterior parts of these anterior muscular im- 

 pressions this groove is traversed by a fine median striation or ridge. 

 In mature specimens, about 5 mm. anterior to the distinct posterior end 

 of the median ridge as just described, curved lateral branches start out 

 from the sides of the ridge antero-laterally, and separate the muscular 

 area into anterior and posterior adductor impressions. The lateral 

 ridges are low and are distinct only between these impressions ; their in- 

 ner posterior edge curves backwards laterally, forming a scarcely visible 

 border for the antero-lateral portion of the posterior aaductor muscular 

 impression, but even this faint border fails more posteriorly. The poste- 

 rior adductor impressions are therefore distinctly defined only along 

 their anterior half, the posterior half having no distinct margin and be- 

 ing usually smooth owing to the generally marked thickening of this 

 portion of the shell. Low broad ridges, fairly parallel to the strong main 



