4 PROCEEDIISrGS OF THE NATIONAL MUSEUM vol. 76 



% mile north of Center Lake School, Bell County (U. S. G. S. Coll. 

 14074) ; Cottonwood Creek, 2.7 miles north-northwest of Hutto, 2 

 miles west-southwest of Montadale, Williamson County (U. S. G. S. 

 Coll. No. 14072) ; near top of hill west of F. V. Browning's ranch 

 house, 3.1 miles south by east of Hutto, Williamson County (U. S. 

 G. S. Coll. 14071) ; 25 feet below top of Austin chalk, on LitUe AVal- 

 nut Creek, 2% miles southwest of Sprinkle, 0.2 mile downstream 

 from the iron bridge of the old Sprinkle road crossing, Travis County 

 (U. S. G. S. Coll. No. 14163). 



In addition to the localities just enumerated, where collections 

 were made, this oyster was observed in a branch east of the public 

 road, 1.5 miles south of Jonah, Williamson County. 



EXOGYRA TIGRINA, new species 



Plate 3 



Description. — Shell small for the genus, inequivalve, broadly sub- 

 ovate in outline; shell wall of moderate thickness. Dimensions of 

 the holotype : Length 56 mm., height 53 mm., thickness 31 mm. 



Left or lower valve much larger than the right, strongly convex, 

 attached at the tip of the beak where only a very small scar of at- 

 tachment is present on most specimens. The valve is rather openly 

 spiral, attaining about 2^ volutions in the holotype. Umbonal ridge 

 pronounced, round-crested, sloping steeply on either side, curved to 

 conform to the spiral twist of the shell. Hinge narrow and curved 

 in the manner normal to the genus : ligamental groove narrow, deeply 

 impressed. Adductor scar of medium size, situated a little above 

 the midheight and toward the rear. 



Surface of left valve smooth for several millimeters back from 

 the beak, beyond which it is ornamented with irregular, round- 

 crested costae of weak to moderate strength; these bifurcate fre- 

 quently on the umbonal ridge, and less frequently on the slopes; 

 about the outer half of the surface of the larger individuals is 

 modified by the more or less prominent development of concentric, 

 imbricating growth lamellae which at their intersections with the 

 costae expand outward in spinelike folds 1 or 2 millimeters high; 

 these folds vary in prominence and are easily broken, none of them 

 being perfectly preserved. The surface is further marked by brown- 

 ish, radiating color bands, which alternate with gray bands; their 

 distribution is similar to that of the costae on the posterior slopes 

 of which they are chiefly develoj^ed ; however, they sometimes occupy 

 the interspaces or the crests of the costae; their distribution was 

 apparently controlled by the costae. This is one of the rare cases 



