53 ^T. Maurice and Claiborne Pelecypoda 53 



showing a tendency to have grooves on the distal i)ortions of the 

 big umbonal costse. 



vSenile forms become edentulous save at either extremity of 

 the hinge. 



Types — Phila. Acad. Nat. Sci., No. 5306. 



Speciniens figured. — Paleont. Mus. Cornell Univ.; Aid. Coll., 

 Johns Hopkins Univ. 



Localities. — Texas : Lee Co. ; Louisiana : St Maurice ; )/z 

 mile S. of Winona ; Hammett's Branch ; 3 miles S. E. of Ne- 

 greet ; Mississippi : Newton ; Wautubbee (Aldr.). Ala. : Clai- 

 borne sands ; base of Claiborne bluff, Lisbon. 



Area vaughani Casey, PI. 22. Figs. 5, 6, 7. 



Area rhoinboidella var. Vaughan, U. S. G. S., 142 ; pi. 3, fig. 8, 1896. 

 A. vatig/iani Casey, Proc. Acad. Nat. Sci. 1903, p. 265. 

 A. vaugliani Sheldon, Palaeontogr. Amer., vol. i, 1916, p. 31, pi. 7, 

 figs. II. 



Casey' s description . — A species quite common in the Lower Claiborne 

 at St. Maurice, La., and allied somewhat ior/ioiiihoidella Lea. It attains a 

 length of more than 22 mm with a height of 12 mm or more, obliquely rhom- 

 boidal, moderatel}- inequilateral, rounded anteriorly and posteriorly and 

 broadly rounded ventrally. It is moderately inflated, the radiating concav- 

 ity at the middle of the umbones almost obsolete and having merely slightly 

 wider intervals between the ribs, the latter 41-43 in number. The hinge- 

 line is long and straight, the teeth becoming larger and very oblique later- 

 ally but well developed throughout, with their sides finely ribbed, giving to 

 each tooth a bipectinate appearance. The area under the beak is ample and 

 l)roadly divaricately striate. This species differs from rhoitiboidella in its^ 

 much larger size, more numerous ribs, rounded ventral edge and many other 

 characters, and may be named vaug/iatii. A fair illustration of it was given 

 by Mr. Vaughan (Bull. Geol. Surv. 142, PI. Ill, fig. 8), in whose honor it is 

 named. A modification of the true rJioinboidella, but still smaller in size, 

 also occurs sparingly at St. Maurice. 



Dr. Sheldon has alread^^ corrected the misconception as re- 

 gards comparative size of the species mentioned. 



This is seemingly a direct off-shoot from rJiomboidella with a 

 more arcuate ventral margin, more even ribbing, more oblique 

 form and comparatively thicker shell substance. There are in- 

 termediate forms, however, ver}- difficult of classification. 



Type. — U. S. Nat. Museum? 



Specimens figured. — Paleont. Mus., Cornell Univ. 



