298 REPORT UNITED STATES GEOLOGICAL SURVEY. 



ference being that of size. But in the absence of full knowledge con- 

 cerning the hinge of either of these forms I prefer at present to leave it 

 under the designation originally given it by Mr. Meek. Compare the 

 description of this form with that of M. holmcsi, immediately preceding; 

 and also compare tigs. 11 a, b, and c, on plate i), with figs. 4 a, b, and c, 

 and figs. 5 a and b, on plate 0. 



Genus PACHYMYA Sowerby. 



Pachymya austinensis Shumard. 



Plate 8, figs. 1 a and b, and plate 5, figs. 7 a and &. 



" Shell very large, length more than double the width, and less than 

 double the thickness ; greatest width near the center, where the shell is 

 very gibbous ; subangulated diagonally from the posterior side of the 

 beak to the anal extremity and sloping to the margins ; posterior slope 

 broad ; sides constricted anteriorly by a broad, shallow depression, which 

 commences some distance below the beaks and extends obliquely back- 

 ward and downward to the base; superior and inferior margins subpar- 

 allel ; buccal end very short, narrowly rounded ; anal end obliquely trun- 

 cate, gaping, angulated at extremity; pallia! margin concave in the mid- 

 dle, rounded before, flattened, incurved, approximate; surface marked 

 with irregular concentric lines of growth. 



"Length, 6.30 inches; width, 2.30; thickness, 3.64. 



" This shell is very nearly related to, if not identical with, P. gigas of 

 Sowerby (Min. Conch, vol. 6, p. 1, pi. 504, 505). The only essential 

 points of difference that I can perceive are that in the foreign shell the 

 beaks are situated nearer the anterior extremity, and the sides do not 

 exhibit the oblique anterior depression which appears to be a constant 

 feature in the Texan fossil." 



Dr. Shumard reports this fossil as from the " Washita limestone on 

 Shoal Creek, near Austin, associated with Terebratula wacoensis, Turri- 

 lites brazoensis, and Ostrea subovata." A large specimen sent to the 

 Smithsonian Institution, from Salado, Bell County, Texas, by Mr. D. H. 

 Walker, measures 22 centimeters in length, a dorsal view of which is 

 shown on plate 8, fig. 1 b, reduced one-half its natural diameter. A 

 young example, also sent by Mr. Walker, is shown on plate 5, figs. 7 a 

 and b. " 



The illustration, fig. 1 a, Plate 8, is a reduced copy from a photograph 

 of one of Dr. Shumard's original drawings, about two-thirds natural di- 

 ameter. The large example first mentioned is a natural cast, and shows 

 a deep, narrow groove passing obliquely downward and a little back- 

 ward, immediately behind the anterior adductor scar, which disappears 

 before reaching the base of the shell, and which, of course, represents a 

 ridge upon the inner surface of the shell, much like what occurs in Pleu- 

 rophorus. The scar of the anterior adductor is large, and situated very 

 near the front margin. 



Pachymya % herseyi (sp. nov.). 



Plate 5, figs. 5 a and b. 



Shell small, elongate-oblong, wider posteriorly than anteriorly, inflated, 

 greatest thickness a little forward of the middle, slightly gaping behind ; 

 beaks depressed, approximate, incurved, placed near the front ; basal 

 margin nearly straight, or only slightly convex; posterior margin trun- 



