894 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 127 
Brachial valve moderately and fairly evenly concave; deepest part at middle; 
sides and anterior deflected moderately toward the brachial valve. Interior with 
delicate cardinal process ; short median ridge and slender adductor ridges, all 5 of 
the latter well developed. 
Measurements in mm.— 
Brachial Hinge 
Length length Midwidth width Thickness 
FHloloty pew. we spelen. 31.4 29.1 36.0 28.8 ? 6.4 
Paratype (117776a).. 28.3 25.6 38.6 37.8 vee 
oy (117772) 5 «0270 25.8 31.4 31.2 5 fee 
Types.—Holotype: 117773; figured paratypes: 117772, 117774a,b, 117775,, 
117776a,b; unfigured paratypes: 117775b-d, 117776c,d. 
Horizon and locality—Whistle Creek formation in Virginia: Along U. S. 
Highway 60 about 14 to 2 miles northwest of Lexington and on Whistle Creek 
northwest of Lexington, Lexington (15’) Quadrangle. 
Discussion.—This species is characterized by its small size for the genus, its 
fairly strong convexity, and the even, gentle concavity of the brachial valve. The 
species is not well known because it is quite rare. The specimens so far col- 
lected indicate some variability in the species particularly in the hinge width. No 
other described species of Macrocoelia is quite like this one. Macrocoelia rotunda 
is generally squarer and more convex and has a more concave brachial valve. 
Macrocoelia plebeia does not approach it in size or convexity. 
MACROCOELIA MAGNA (Butts) 
Plate 231, C, figures 9-14; plate 232, A, figures 1-7; plate 235, E, figures 17-21; 
plate 236, C, figure 13; plate 230, C, figure 29 
Rafinesquina magna Burts, Virginia Geol. Surv. Bull. 52, p. 85, pl. 86, figs. 1-4, 1942. 
Shell large for the genus in adult form, moderately convex, wider than long 
with the greatest width at the hinge in the young but near the middle in old 
adults. Anterior margin broadly rounded; lateral margins generally oblique. 
Surface marked by costellae of unequal size, the costellae at the posterior very 
different in size, the larger ones separating I to 3 finer ones. Costellae on the 
anterior part of the valves becoming more uniform in size, but 1 to 3 finer cos- 
tellae may be seen between the coarser ones although this is not so frequent as 
at the posterior of the valves. Finer costellae cancellated by strong fila. A few 
oblique wrinkles occurring on each side of the beak along the posterior margin. 
Pedicle valve moderately convex in lateral profile with the greatest convexity 
located slightly anterior to the middle. Lateral slopes moderately steep ; postero- 
lateral slopes to cardinal extremities gentle. Interarea short; approximately 
orthocline. Foramen minute, pseudodeltidium short, not strongly arched. 
Brachial valve moderately and fairly evenly concave; posterior two-thirds 
gently concave but anterior third somewhat abruptly bent anteriorly. Cardinal 
extremities very slightly deflected in an anterior direction. Interarea short; 
chilidium short and low. 
