298 EEPORT 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. holmesi, immediately preceding ; 

 and also compare figs. 11 a, h, and c, on plate 9, with figs. 4 a, h, and c, 

 and figs. 5 a and h, on plate 6. 



Genus PAOHYMYA Sowerby. 



PACHY3MYA AUSTINENSIS Shumard. 



Plate 8, figs. 1 a and Z>, and plate 5, figs. 7 a and 6. 



" 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; pallial 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. gif/as 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 Terehratula wacoensis, Turri- 

 lites hrazoe7isis, and Ostrea suhovcitaP 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 &, reduced one-half its natural diameter. A 

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

 and 6. 



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 disapi^ears 

 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 Fleu- 

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

 near the front margin. 



PACHYMYA ? HEESEYI (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, api^roxunate, incurved, placed near the front ; basal 

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



