stanton.] MACTRID.E. 121 



of the internal characters, it is much more probable that they are 

 really very distinct. So far as regards their form and general appear- 

 ance, they seem only to differ in having the anterior side rather more 

 produced and sometimes wider. 



"From this general resemblance, however, I have, in the absence of 

 any knowledge of the nature of its cardinal teeth or pallial line, ven- 

 tured to refer it provisionally to the same section of the Mactra group 

 to which Tellina nitidula is now believed to belong, that is, to Cymbo- 

 phora Gabb. I should have been inclined to refer it to Macoma or 

 Gastrana were it not for the impressions of lateral teeth seen before 

 and behind the beaks in the casts. 



u Locality and position. — Whitish Cretaceous sandstone, East Canyon 

 creek, Wasatch range, and near Coalville, Utah." 



At Coalville this is one of the most abundant species in the strata 

 below the principal coal bed, but they are all in the same state of pres- 

 ervation as the types. At first I was inclined to doubt whether the 

 two forms represented by Meek's figures are really the same species, 

 but after comparing a large series I am unable to find any constant 

 differences by which they may be separated. Several specimens of the 

 equilateral form show impressions of cardinal teeth like those of 

 Mactra. 



Mactra emmonsi Meek. 



PL xxvn, Figs. 9-13. 



• Mactra ? emmonsi Meek, 1877, U. S. Geol. Expl. 40th Parallel, vol. iv, Pt. 1, p. 153, 

 PI. 15, Fig. 8. 



" Shell small, oval-subtrigonal, rather compressed, longer than high, 

 nearly or quite equilateral, or with anterior side slightly longer than 

 the other; basal margin forming a semielliptic curve; anterior mar- 

 gin narrowly rounded below the middle; posterior margin somewhat 

 broader, most prominent and abruptly rounded or obtusely subangular 

 below, and very faintly subtruncated obliquely above; dorsal margin 

 sloping before and behind the beaks, the anterior slope being greater, 

 with a concave outline; beaks nearly central, or sometimes placed a 

 little behind the middle, rather depressed, and incurved with very slight 

 obliquity; posterior umbonal slope very obscurely angular from the 

 beaks to the posterior basal extremity. Surface only marked by fine 

 obscure lines of growth. (Hinge and other internal characters un- 

 known.) 



"Length, 0.45 inch; height, 0.30 inch; convexity, 0.17 inch. Some 

 specimens of apparently the same species are nearly double the size of 

 that from which the above measurements were taken, and some of the 

 smaller ones are proportionally a little shorter." 



The figured type is a small sandstone cast somewhat distorted by 

 pressure, and with the beak partially concealed in the matrix, so that 

 it was drawn less prominent and more obtuse than it really is. It was 



