AKT. 18 NEW MOLLUSKS FROM TEXAS — STEPHENSON 5 



in which the color markings of the living shell are preserved in the 

 fossil state. These color markings are present on most of the speci- 

 mens collected from the six localities, their apparent absence on some 

 individuals being due to weathering and leaching. 



Upper or right valve flatly spiral, operculiform, slightly convex 

 anteriorly, becoming concave 'posteriorly, fitting neatly within the 

 margins of the left valve. Surface ornamented with numerous sharp- 

 edged lamellae, separated by deep depressions, which are closely 

 spaced on the anterior part of the shell, and more widely spaced 

 posteriorly; on the type there is also an insipient development of 

 irregular costae which produce the same sort of spinelike folds 

 where they cross the concentric lamellae, as are present on the left 

 valve, but costae are absent on some specimens; color bands are 

 wanting on the right valve. Ligamental groove narrow and deeply 

 impressed. 



Remarks. — This species is about the same size and has about the 

 same general form as Exogyra laeviuscula Roemer, an upper Austin 

 chalk species. Roemer's species is smooth, with no indication what- 

 ever of radiating costae, and is entirely wanting in color bands; it 

 is also broader in the umbonal region, has a less prominent and more 

 rounded umbonal ridge, and is not quite so openly coiled. 



Like most other oysters this species varies considerably in out- 

 line and form and in the coarseness of the costae. In some indi- 

 viduals the costae are considerably finer than they are in the type. 



So far as known the species is confined to a zone only a foot or 

 two in thickness. It was first collected in 1894 by Dr. Robert T. 

 Hill at the two localities (see below) which are south of Colorado 

 River, in Travis County. 



Ty'pe.—QdX. No. 73660, U.S.N.M. From Little Walnut Creek, 214 

 miles southwest of Sprinkle, Travis County. (See below.) 



Distribution. — Upper part of Austin chalk: Onion Creek, half 

 a mile above Bluff Springs, Travis County (Hill Coll. No. 54, 

 U.S.N.M.) ; Williamson Creek between the upper and lower Lockhart 

 road crossings, Travis County (Hill Coll. No. 51, U.S.N.M.) ; Little 

 Walnut Creek, 0.2 mile downstream from the iron bridge of the 

 old Sprinkle road crossing, 35 feet below top of Austin chalk, 214 

 miles southwest of Sprinkle, Travis County (U. S. G. S. Coll. 14164) ; 

 public road, 1.2 miles south of Jonah, Williamson County (U. S. 

 G. S. Coll. 13809) ; on the upthrow side of a small fault, in ravine 

 2% miles west-northwest of Sparks, % mile north of Center Lake 

 School, Bell County (U. S. G. S. Coll. 14073) ; Little Elm Creek, 

 2.4 miles northeast of Temple, Bell Countv (U. S. G. S. Coll. 

 13822). 



