286 EEPORT UNITED STATES GEOLOGICAL SUEVEY. 



tremely inflated with age (when the back of the shell becomes broad and 

 concave), sometimes reaching a length of 19 or 20 centimeters, a height 

 of 10 or 11 centimeters, and a thickness about equal to the height. The 

 usual dimensions, however, are about one-quarter less. 



This species was designated by Mr. Meek in a foot-note to a list of the 

 fossils collected by the surveying parties during the year 1870, on page 

 297 of Ann. Eep. U. S. Geol. Surv. Terr, for 1870. Just previous to 

 his death he had also prejjared the drawings of the type-specimens repre- 

 sented by figures 1 a and 6, plate 2, which are there reduced to three- 

 quarters the original size. 



Position and locality. — In the list of fossils referred to, the locality is 

 given as " Cache la Poudre Eiver, near Greeley, Denver and Pacific Eail- 

 road, Cret. No. 3." During a personal examination of that region in 

 1877, 1 found what I presume is the original locality to be about 15 miles 

 west of Greeley and about 6 miles south of Fort Collins. The strata of 

 that portion of the Cretacious series, however, are no doubt exposed at 

 various localities between the Cache la Poudre and the Big Thompson. 

 At the first-named locality numerous specimens of this species were found 

 with both valves in place, weathered out of the harder sandstone concre- 

 tion distributed in the softer strata, where they are associated with 

 several other species characteristic of the divisions 4 and 5 of the Upper 

 Missouri section. 



Genus BAEBATIA Gray. 

 Baebatia Coalyillensis White. 



Plate 6, figs. 2 a and &. 



Area f coalvillensis White, 1876, Powell's Eep. Geol. Uinta Mts. p. 115. 



Shell not large, moderately gibbous, transverse length from two-fifths 

 greater to nearly twice as great as the height; beaks depressed, situated 

 near the anterior end ; umbones broad, not prominent ; anterior end 

 rounded or subtruncate; base usually nearly straight, but sometimes 

 slightly convex, and sometimes a little emarginate about the midlength ; 

 postero-basal border rounded upward to the posterior extremity, which is 

 abruptly rounded up to the downward-sloping, nearly straight postero- 

 dorsal border, the latter forming a rounded obtuse angle with the cardinal 

 border ; hinge equal in length to about two-thirds the entire length of 

 the shell, consisting of a moderately slender hinge-plate bearing numer- 

 ous transverse teeth with about equal spaces between them. The pos- 

 terior teeth have an oblique direction downward and a little forward, 

 which obliquity diminishes toward the front, so that the teeth from 

 about midlength of the hinge to the center of the beak are directly trans- 

 verse. These central transverse teeth are a little narrower than those 

 farther back, but the two or three teeth in front of the center of the beak 

 are larger than any of the others, and a little curved. 



Area apparently nearly obsolete, or at least it is very narrow. Internal 

 markings unknown. A slight depression or flattened space upon the 

 outer surface extends from the umbo of each valve to its base, meeting 

 there the straight or shghtly emarginated portion of the basal margin 

 before mentioned. 



Surface marked by the ordinary lines of growth, and also by five radi- 

 ating lines, which are often obscure. 



Length of an example a little above the ordinary size, 5 centimeters ; 

 height, 33 millimeters. 



In the original publication of this species {loc, cit), 1 was a little mis- 



