120 COLORADO FORMATION AND ITS INVERTEBRATE FAUNA, [buix.106. 



Externally this species has a close resemblance to certain forms of 

 Pleuromya, and I was inclined to refer it to that genus, although it is 

 not reported as occurring in beds later than the Lower Cretaceous. On 

 further examination a part of the hinge of a left valve was exposed, 

 revealing a prominent conical tooth under the beak. So far as it is 

 known, therefore, the hinge is like that of Liopistha. The surface is 

 not well enough preserved to show whether it was marked by the obscure 

 radiating lines of granules or minute spines that usually appear on 

 species of the Psilomya section of this genus, but this feature seems to 

 be wanting in Liopistha (Panopwa) frequens Zittel, a species from the 

 Gosau beds that is very closely related to ours. Another species, 

 described above under the name Liopistha (Psilomya) concentrica, has 

 very nearly the same surface ornamentation as L. elongata, but its 

 form is more nearly like that of the typical species of the genus. It is 

 worthy of remark that the three species herein described are the only 

 American ones that have been referred to the subgenus Psilomya, and 

 they all come from approximately the same horizon. 



Locality and position. — Upper Kanab valley, Utah, from concretions 

 in shale about 350 feet above the base of the Cretaceous section. 



MACTRID^E. 

 Genus MACTEA Linnaeus. 

 Mactra (Cymbophora?) utahensis Meek. 

 Plate xxvn, Figs. 16 and 17. 



Mactra (Cymphobora) utahensis Meek, 1877, U. S. Geol. Expl. 40th Parallel, vol. IV, 



Pt. 1, p. 155, PI. 15, Figs. 9, 9a, 9&. 



" Shell subovate, moderately convex; anterior margin rounded; pos- 

 terior margin narrower, and rather abruptly rounded, or sometimes ap- 

 parently slightly truncated, being most prominent below; basal mar-, 

 gin forming a semielliptic or semiovate curve, being sometimes more 

 prominent anteriorly ; dorsal outline sloping from the beaks toward the 

 extremities ; beaks moderately prominent, very nearly central, and in- 

 curved with little obliquity; umbonal slopes merely rounded, and not 

 terminating in a flexure of the posterior basal margin. Surface ap- 

 parently merely marked with fine, obscure, irregular lines of growth. 

 Hinge merely known to possess linear anterior and posterior lateral 

 teeth. Ligament and internal characters unknown. 



" Length, 1.35 inches; height, 0.90 inch; convexity, about 0.50 inch. 



" The specimens of this shell in the collection agree so nearly with a 

 form described by the writer in connection with Dr. Hayden, from the 

 Upper Cretaceous beds on Deer Creek, near the North Platte, under 

 the name Tellina nitidula, that I was at one time inclined to think 

 they might belong to a variety of that species. Still, as they are merely 

 casts, giving but a limited knowledge of the hinge, and showing nothing 



