STANTON.] 



MYID^. 123 



having the following dimensions, respectively : Length, 30 and 38 n,m ; 

 height, 21 and 22 mm ; convexity (of single valve), 5 and 4 mm . 



The left valve has a medium-sized cartilage pit with a A-shaped 

 cardinal tooth in front of it. Some of the casts show impressions of small 

 lateral teeth. The anterior muscular impression is ovate or subcircular, 

 the posterior, elongate ovate. Pallial sinus rather broad, rounded at the 

 extremity, and horizontal. The structure of the hinge is determined 

 from internal casts that do not show all the features clearly, so that it 

 is impossible to say whether the species is* a Mactra or some other 

 allied genus. 



I was at first inclined to identify these fossils with Tellina? isonema 

 which they closely resemble in general form. They differ from that 

 species, however, in that their surface is much smoother, lacking the 

 regular concentric lines and also the angular posterior umbonal slope. 



Locality and position. — In the Pugnellus sandstone at many localities 

 in Huerfano park, Colorado. 



MYID^E. 



Genus COEBULA Brugutere. 



I/ORBULA SUBTRIGONALIS M. & H. 



PI. xxvn, Figs. 7 and 8. 



Corbula siibtrigonalis Meek and Hayden, 1856, Proc. Acad. Nat. Sci. Phila., p. 116. 



Corbula perundata Meek and Hayden, ibid. 



Corbicula crassatelliformis Meek, 1871, Ann. Rept. U. S. Geol. Sur. Terr, for 1870, p. 



315. 

 Corbula tropidophora Meek, 1873, idem for 1872, p. 514. 

 Corbula subtrigonalis White, 1879, idem for 1877, p. 170; 1880, idem for 1878, p. 80, 



PI. 25, Figs. 6a-f ; 1883, 3d Ann. Rept. U. S. Geol. Sur., p. 442, PL 19, Figs. 



10-13. 

 Compare Corbula perangulala Wliiteaves, 1885, Cont. to Can. Palaeont., vol. i, p. 9, 



PI. 1, Fig. 5a, b, PI. 2, Fig. 1. 



Shell large, transversely elongate- triangular; moderately convex, sub- 

 equivaive, the right valve being slightly longer and more convex than 

 the other; anterior end short, obliquely sub truncate above, regularly 

 rounded below to join the ventral margin, which forms a regular, 

 gently convex curve; posterior end narrow, greatly produced, subangu- 

 lar at the extremity; posterior dorsal margin straight or very slightly 

 convex, forming approximately a right angle with the truncate anterior 

 end ; beaks prominent, incurved ; posterior umbonal slope very promi- 

 nent and angular in the left valve, more rounded in the right. A 

 broad area immediately in front of the umbonal ridge is flattened, while 

 the portion of the shell between it and the dorsal margin is abruptly 

 deflected. Surface ornamented by fine lines of growth and a few larger 

 concentric ridges. 



Length of a large specimen, 33 mm ; height, 21 mm ; convexity of a single 

 left valve, 7 mm . 



The specimens here described were collected from coal-bearing sand- 



