50 Bulletin 31 50 



the exterior of the shell seems never to have been ribbed at 

 all, and to such specimens de Gregorio has given the varietal 

 name ig7itis. Some well-preserved specimens show clear-cut ribs 

 and are termed "var. striatus" by de Gregorio. The most cuneate 

 forms he styles ' 'var. percuneatus' ' . 



This author well remarks: "These four forms pass from one to the 

 other, in such a way that it is difficult to determine them when one has be- 

 fore him a large number of samples. Nevertheless I have deemed it advan- 

 tageous to make them known, for in a hasty examination, one might take 

 them to be different species (p. 195. Mon, Faun. Eoc. Ala). 



Well preserved specimens, generally young, show in the early 

 stages of growth a fine, crowded, slightly irregular concentric 

 sculpturing. The ribs are most pronounced generallj^ anteriorlj', 

 but are very variable in size and strength in different individuals ; 

 slightly bent backwards medially ; with anterior margins higher 

 than the posterior and retaining this feature to the very anterior 

 margin of the shell. 



Judging from the specimens before me it would appear that 

 the more circular forms, with but slight radiating sculpture, the 

 minor type of Lea, is the older and the more prevalent in the St. 

 Maurice stage. Some specimens from Hammett's Branch show 

 some variation, but all are very small. Those from the Ruston 

 well (400 ft. ) are more nearly normal minor. The species is rare 

 in lyouisiana, known to me from Texas, by one small, eroded 

 valve, and occurs at Wautubbee, Miss., var. wautubbeana, n. 

 var., in a thick, small, heavy form with so far as observed no fine 

 umbonal concentric sculpturing, but with heavier lines of growth 

 extending over more of the surface of the shell, recalling to mind 

 the surface features of Glycymeris lisbonensis while very small, 

 though without the anterior and posterior radii. Very small 

 specimens of this variety occurs at the 1200 ft. level in the Ruston 

 Well. 



7>;^^.— Phila. Acad. Coll. 



Horizo7is. — St. Maurice and Claiborne Stages. 



Specimen figures. — C. U. Pal. Mus. 



Localities. — Hurricane Bayou. Houston Co., Tex. (one valve 

 only), Tex. St. Mus. ; Hammett's Branch ; Ruston' well, La. , 



