151 St. Maurice and Ci^aiborne Pelecypoda 151 



circumstances it seems unwise to attempt to refer the form here 

 described as bastropoisis to Gahh' s yoakumi . 



dementia merccnaroidea Aldrich, PI. 47, Figs. 8-1 1 



Dosinia vierceuaroidea Aid., Jr. Cinn. Soc. Nat. Hist., 1887. p. 82 ; and 



Bull. Am. Pal. vol. 2, '97, p. 172, pi. 2, fig. 10. 

 Meretrix mercenaroidea Dall, Trans. Wag., Ill, 1903, p. 1234. 



Aldrich' s original description. — Shell orbicular, moderately com- 

 pressed, concentrically finely striated, nearly smooth upon the umbo. Sub- 

 stance of the shell thin, thickening towards the margins ; lunule rather 

 long and narrow, beak curved toward the lunule, small and anterior to the 

 center: hinge line rather long. Teeth in left valve rather prominent, erect, 

 central one subtriangular. Ventral margin smooth. 



Length, 1.4, Breadth 1.15, Thickness .6 of an inch. 



Locality. — Upper Landing at base of Claiborne Bluff. 



This species is more rotund than the recent D. concentrica Gmel. and 

 much thicker through the umbones. Only two specimens found. 



This differs from bastropensis in form and dentition. 



Dall has questioned the propriety of referring this species to 

 Dosinia and has suggested that it has a decidedly Meretrix ap- 

 pearance. So it would seem, but on examining our specimens 

 we find no trace, of an anterior lateral tooth and no trace of either 

 lunule or escutcheon. One usually takes fragments of this shell 

 for Meretrix trigoniata for the markings are similar, but the 

 shape of the shell and the hinge characters distinguish the two 

 at once. Most dementias are thinner, with more weakly devel- 

 oped posterior cardinal and with more ascending pallial sinus. 

 But there was a time when this genus was first differentiating 

 from MeretrixA\ke. forms and this species may be the initial re- 

 sult of such a process. 



Tjr/'*?.— Aldrich Coll., Johns Hopkins Univ^ 



Horizon. — St. Maurice Eocene. 



From the base of the bluff just above the "Upper Landing," 

 Claiborne, Ala. 3 miles W. N. W. of Orangeburg, S. C. 



*For illustration of right valve and hinge of left see these Bulletins 

 vol. 2, pi. 2. 



