MARYLAND GEOLOGICAL SURVEY 311 



becoming smaller towards the posterior end; margin crenulated." Con- 

 rad, 1831. 



Ribs well spaced, almost uniformly thin, but not uniformly recurved; 

 deeply grooved beneath the recurved portion ; beak profoundly curved an- 

 teriorly; muscle impressions small, subequal; pallial sinus a mere notch; 

 marginal crenulation minute. 



Length, 29 mm. ; height, 26 mm, ; diameter, 10 mm. 



Occurrence. — St. Mary's Formation. St. Mary's River. 



Collections. — Maryland Geological Survey, Johns Hopkins University. 



Subfamiiy MERETRICIN^. 



Genus MACROCALLISTA Meek. 



Macrocallista marylandica (Conrad). 

 Plate LXXIV, Figs. 1, 2. 



Cytherea Marylandica Conrad, 1833, Amer. Jour. Sci., voL xxiii, p. 343. 



Cytherea Marylandica Conrad, 1838, Fossils of the Medial Tertiary, p. 1.5, pi. ix, 



fig. 1. 

 Dione Marylandica Conrad, 1863, Proc. Acad. Nat. Sci. Phila., vol. xiv, p. 575. 

 Dione marylandica Meek, 1864, Miocene Check List, Smith. Misc. Coll. (183), p. 9. 

 Dione Marylandica Whitfield, 1894, Mon. xxiv, U. S. Geol. Survey, p. 74, pi. xiii, 



fig. 1. 

 Macrocallista albaria Dall, 1903, Trans. Wagner Free Inst. Sci., vol. iii, pt. vi, p. 



1253 (in part?). 

 Macrocallista [Chionella] marylandica Dall, 1903, Trans. Wagner Free Inst. Sci., vol. 



iii, pt. vi, p. 1255. 



Description. — "Shell obtusely ovate, smooth, thick; umbo obtusely 

 rounded posteriorly; lunule ovate-acute and slightly impressed; hinge 

 with the anterior tooth very robust.'' Conrad, 1833. 



Shell thick, ponderous, moderately inflated; surface polished, crossed 

 by faint concentric undalations; anterior extremity gently rounded, pos- 

 terior extremity acutely rounded ; hinge area ponderous ; anterior muscle 

 impression profound; pallial margin distinct; pallial sinus not profound. 



The young of this species is much thinner, flatter and longer in propor- 

 tion to the height than the adult forms, having in fact the shape of 

 M. albaria, and in Maryland at least has often been called M. alharia. 

 Whether M. albaria from Virginia be really merely the young of M. mary- 

 landica or not I have no means of telling. I know of no authentic speci- 

 mens of if. albaria from Maryland, 



