470 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 127 
to slightly concave and with gentle anterior slopes but moderately steep pos- 
terolateral slopes. Interarea strongly curved, apsacline. Beak prominent and 
incurved. 
Brachial valve about equal in depth to the pedicle one, with a fairly strongly 
convex lateral profile with the posterior half swollen. Anterior profile strongly 
convex and with steep lateral slopes. Fold imperceptible; the entire median 
region somewhat swollen with an inflated umbo and steep posterolateral and 
lateral slopes. Anterolateral slopes moderately steep. Interarea curved, apsacline. 
Measurements in mm.—Holotype, length 11.6, brachial length 11.0, width 14.7, 
hinge width 10.5, thickness 8.1. 
Types—Holotype: 110658a; figured paratype: 110658b. 
Horizon and locality—tLincolnshire formation (Hogskin member) in Ten- 
nessee: I mile southeast of Washburn, Dutch Valley (T.V.A. 154-SE) Quad- 
rangle; Inskip, Fountain City (T.V.A. 146-SW) Quadrangle; Eidson, Kyles 
Ford (T.V.A. 170-SE) Quadrangle. 
Discussion.—This species is characterized by its generally small size and its 
finely costellate exterior. It differs from M. similis, also occurring in the Hogskin 
shale, by its more swollen valves, particularly the pedicle valve, and its finer 
ornamentation. 
MIMELLA EXTENSA Cooper, new species 
Plate 86, A, figures 1-21; plate 90, A, figures I-15 
Shell of about medium size for the genus; wider than long; hinge not equal to 
the greatest shell width which is at about the middle, but unusually wide never- 
theless. Cardinal extremities obtuse to acute, frequently with pointed ears. Lat- 
eral margins gently rounded; anterior margin broadly rounded; anterior com- 
missure broadly uniplicate. Surface costellate, 8 to 10 costellae in the space of 
5 mm. at the front margin; costellae narrowly rounded and somewhat elevated 
with interspaces about equal to the width of the costellae; costellae marked by 
innumerable small pores or exopunctae with elevated rims. 
Pedicle valve moderately convex in outline with the greatest curvature located 
in the posterior third; umbonal region gently convex; beak erect; sulcus origi- 
nating near the middle, poorly defined and very shallow. Flanks slightly convex 
and the slopes to the cardinal extremities and lateral margins gentle. Interarea 
long, nearly procline along the cardinal margin but curving to an apsacline posi- 
tion near the beak. 
Brachial valve fairly strongly convex in lateral profile, with the maximum 
curvature near the middle; strongly convex in anterior profile but with the shell 
near the margins slightly flattened. Sulcus originating on the umbo and extend- 
ing beyond the middle of the valve as a shallow and narrow depression disap- 
pearing near the front. Fold visible only as a wave in the anterior commissure 
toward the brachial valve. Median portion of valve swollen and with steep 
slopes to the cardinal extremities but more gentle slopes to the anterolateral and 
anterior margins. 
Pedicle interior with small teeth having deep fossettes. Muscle field large but 
