Maryland Geological Survey 539 



Fort Pierre group of the Western Interior. A. exigua is a little larger 

 than its eastern relative, quite a little higher relatively, and more inflated. 



There are no generic characters by which this species can be separated 

 from Area, subgenus Barbatia. Breviarca was used by Conrad to include 

 subcircular or cordiform species with numerous minute cardinal teeth 

 arranged in a broad arc. 



Occurrence. — Matawan Formation. Ulmstead Point (?). Mon- 

 mouth Formation. Bohemia Mills, Brightseat, Brooks estate, McNeys 

 Corners. 



Collections. — Maryland Geological Survey, Philadelphia Academy of 

 Natural Sciences, New Jersey Geological Survey, U. S. National Museum. 



Outside Distribution. — Matawan Formation. Woodbury clay, New 

 Jersey. Ripley Formation. Exogyra costata zone, Hardeman County, 

 Tennessee. Pierre Shales. Western Interior. Of. Area exigua Meek and 

 Hay den. 



Arca (Barbatia) uandi n. sp. 

 Plate XXI, Figs. 5, 6 



Description. — Species described from an unusually well-preserved cast 

 of double valves. Shell of moderate size, subrhomboidal in outline ; disk 

 sinuated by a depression originating at the tips of the umbones and 

 becoming increasingly broad and shallow toward the ventral margin; 

 posterior area well differentiated by an obtuse angulation extending from 

 the umbones to the posterior basal margin; anterior area less sharply 

 defined ; length of hinge line slightly exceeding three-fourths of the total 

 length ; base line made slightly flexuous by the umbonal sinus ; anterior 

 lateral margin rounding rapidly into the base; posterior lateral margin 

 obliquely truncate; cardinal area rhomboidal, striated with two or three 

 oblique grooves; surface of cast sculptured with about forty linear stri- 

 ations radiating from the umbones to the base, but absent upon the poste- 

 rior and the greater part of the anterior area; characters of hinge 

 unknown. 



Dimensions. — Altitude 19 mm. ; latitude 39 mm. ; maximum diameter 

 18.1 mm. 



