FOSSILS OP THE CLINTON GROUP. 577, 



valve this thickening extends chiefly along the hinge margin ; elsewhere 

 it is moderate, being most pronounced over the muscular impression 

 area, and at times a little within 1 the anterior and lateral borders of the 

 valve. From the anterior. end of the cardinal tooth a ridge extends for- 

 wards which is strong and well defined between the posterior adductor 

 impressions,' and quite high in older individuals, and lower and narrower 

 between the anterior adductor impressions. The posterior impressions 

 are smaller and are separated from the anterior ones by a low ridge running 

 from the median ridge antero-posteriorly, the two branches of the median 

 ridge making an angle of about one hundred and thirty-five degrees. The 

 anterior border of the anterior adductor impressions is more or less par- 

 allel to the ridges just described. The lateral boundaries are nearly 

 enough parallel to the median ridge .to give all the adductor impressions , 

 a general rhomboid shape. The anterior and lateral outlines' of y the 

 muscular area aire sometimes indifferently indicated in these valves; at 

 other times, especially in older specimens, they are very distinct. The 

 degree of distinctness of the muscular impressions is evidently a question 

 of the degree of thickening of the shell between and around the points of 

 attachment of the muscles. Under favorable circumstances other mark- 

 ings may be seen. Along the exterior side of the lateral border of the 

 muscular area this border gives rise to lateral branches which soon dis- 

 appear laterally, and which give rise to a number of depressions along 

 the borders of the muscular area, especially along the anterior adductor 

 impressions. These are the ovarian markings. From the antero-lateral 

 angles of the muscular area the " anterior 1 , and the lateral borders extend 

 forwards towards the margin of the shell. A third, usually smaller, ridge 

 extends between them. The anterior border of the muscular area and its' 

 anterior continuatio as describe a sort of semi-circular figure. The grooves 

 between these ridges branch a little anteriorly. These are the vascular 

 markings. 



The pedicle valve has a muscular area whose anterior and lateral 

 edges are indicated by the line of greatest thickening of the shell in this 

 region, posteriorly this border rises suddenly into a vertical wall support- 

 ing the teeth. The posterior 'end of the muscular area is quite thin, often 

 permitting the radiating striae to be seen. The thickening increases 

 more or less gradually anteriorly. Fine striae parallelxto the anterior and 

 lateral margins traverse the area, and may be regarded as striae of growth 

 with reference to these margins. Two elevations, usually low and indistinct, 

 becoming more evident anteriorly, traverse the muscular area, dividing it 

 into three long triangular areas, of which the central area is a little larger 

 than the lateral ones. Corresponding to these divisions of the area, the 

 anterior border of the area is often more or less roundly triangulate. It 

 is difficult to determine to what extent the middle triangular area repre- 

 sents the adductor muscular impressions; "perhaps a part of this area was 

 occupied by other muscles which have not left separate impressions. The 



37 G. O. 



