234 SYSTEMATIC PALEONTOLOGY 



" This shell is small. A large ventral valve measures, length 7 mm. : 

 width, 6 mm.; a dorsal valve, length 6 mm.; width, 6.5 mm." Walcott. 



Occurrence. ANTIETAM SANDSTONE. Mouth of Little Antietam 

 Creek near Eakles Mills, one mile east-southeast of Smithsburg and other 

 localities in Washington County, Maryland. 



Collections. Maryland Geological Survey, U. S. National Museum. 



Family DISCINIDAE 



Genus ORBIGULOIDEA D'Orbigny 



ORBICULOIDEA LAMELLOSA (Hall) 



Plate XLVIII, Fig. 12 



Orbicula lamellosa Hall, 1847, Pal. New York, vol. i, p. 99, pi. xxx, fig. 10 



(not Broderip, 1833). 

 Orbiculoidea lamellosa Hall and Clarke, 1892, Pal. New York, vol. viii, pt. 1, 



pi. 4E, fig. 12. 

 Orbiculoidea lamellosa Weller, 1903, Geol. Surv. New Jersey, Pal., vol. iii, 



p. 147, pi. ix, figs. 1, 2. 



Description. " Shell depressed-conical, nearly circular in outline, the 

 apex of the brachial valve situated about one-third of the breadth of the 

 shell from the margin. Surface marked by rather irregular, elevated, 

 more or less lamellose, concentric lines, the grooves between the lines 

 being rather wider than the ridges themselves. 



" The dimensions of a nearly perfect brachial valve are : Length, 

 9.5 mm. ; width, 8.75 mm. ; convexity, 2.5 mm." Weller, 1903. 



Occurrence. CHAMBERSBUEG LIMESTONE (Christiania bed). Penn- 

 sylvania and Virginia. Trenton limestone of Middleville, etc., New York, 

 New Jersey, and Ontario. 



Collection. U. S. National Museum. 



Family ACROTRETIDAE 

 Genus CONOTRETA Walcott 

 CONOTRETA RUSTI Walcott 



Plate L, Figs. 1-5 



Conotreta rusti Walcott, 1890, Proc. U. S. Nat. Mus., vol. xii, p. 365, figs. 1-4. 

 . Conotreta rusti Hall and Clarke, 1892, Pal. New York, vol. viii, pt. i, p. 104, 

 pi. 4K, figs. 16-21. 



