22(3 UNITED STATES GEOLOGICAL SURVEY OF THE TERRITORIES. 



ridge extending down the anal side. The pallial impression is moderately 

 distinct, and provided with a nearly oblong, slightly tapering sinus, which is 

 raised a little above the horizontal axis of the shell, and subtruncated or obtuse 

 at the extremity ; it extends about four-ninths of the distance from the anal 

 border toward the anterior margin of the shell. 



I have not yet been able to see the hinge of this species, but at first 

 referred it provisionally to the genus Tellina, from external characters. 

 Although still in doubt respecting its true relations, I am now inclined to think 

 it presents more the appearance of a Thracia or a Corymya, and consequently 

 remove it for the present to the former of these genera. 



It resembles the last of the preceding species very nearly, but is larger 

 and more compressed, and its basal margin more warped. It also differs in 

 having a thicker shell, which shows, on exfoliated inner laminae, faint traces 

 of minute, fine, radiating striae. 



Professor Agassi/, describes from the Upper Oolite a rather closely-allied 

 form under the name of Corymya Studeri (fitud. Grit, sur les Moll. Foss., 269, 

 tab. 35.) Our shell, however, appears to be a little less gibbons over the 

 umbones, and has a more triangular posterior muscular impression. It is 

 quite probable that this and the last of the foregoing species may have to be 

 removed from the genus Thracia, and called Corymya gracilis and C. Prou/i. 



Locality anal position. — Same as last. (Collection of Dr. Prout, Saint 

 Louis, Mo.) 



