490 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 127 
costellae. It is also featured by a fairly narrow uniplicate wave in the anterior 
commissure suggestive of M. sulcata. It differs from that species in having a 
strongly swollen umbo and less convex pedicle valve. 
MIMELLA sp. 1 
Plate 91, D, figures 19-24 
Shell of medium size, subrectangular in outline with the hinge narrower than 
the greatest width which is at the middle. Anterior commissure rectimarginate. 
Sides very gently rounded. Cardinal extremities prominent. Surface multicos- 
tellate, costellae uneven, crowded, about II in 5 mm. at the anterior margin. 
Pedicle valve gently convex in lateral profile with the posterior half convex 
and the anterior half somewhat flattened. Anterior profile very gently and 
broadly convex. Umbonal and median regions gently swollen; anterior region 
flattened. Lateral and posterolateral slopes long and gentle. Interarea long and 
strongly apsacline ; beak gently incurved. 
Brachial valve evenly and moderately convex in lateral profile with the maxi- 
mum convexity at about the middle. Anterior profile fairly strongly convex with 
long but moderately steep lateral slopes. 
Measurements in mm.—117056a, lerigth 16.2, brachial length 15.9, width 20.4, 
hinge width 18.0, thickness 10.4. 
Figured specimens.—117056a,b. 
Horizon and locality—Elway formation in Tennessee: On U. S. Highway 
25W, 4.7 miles south of Clinton, Powell Station (T.V.A. 137-SE) Quadrangle. 
Discussion.—This species suggests M. laticardimia but has a few differences. 
The pedicle valve is somewhat more swollen, the interarea is more apsacline, the 
cardinal extremities less obtuse, and the umbo faintly sulcate. Only two speci- 
mens are available, both of which are imperfect ; consequently, no specific name 
has been applied. 
MIMELLA sp. 2 
Plate 91, G, figures 29-34 
Shell small to medium size for the genus, wider than long, hinge narrower 
than the greatest shell width at the middle. Sides rounded, anterior margin 
broadly rounded; anterior commissure rectimarginate. Surface multicostellate, 
costellae fine, closely crowded, numbering about 17 in 5 mm. at the front margin. 
Pedicle valve gently convex in lateral profile; anterior profile broadly convex 
but the median ridge rounded and with long, gentle lateral slopes. Umbonal 
region somewhat swollen ; median region gently inflated to form a poorly defined 
fold. Lateral areas flatly convex. Interarea strongly apsacline, gently curved. 
Brachial valve gently convex in lateral profile; anterior profile gently and 
broadly convex, the median region slightly depressed. Umbo marked by a shal- 
low sulcus that extends from the beak to the anterior margin. Flanks bounding 
sulcus gently swollen; posterolateral extremities prominent, marked off by long 
gentle slopes. 
