474 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 127 
M. globosa is often gently sulcate in the posterior half, but the sulcus disappears 
before the anterior margin is reached. This is unlike M. imperator which has a 
stronger and more prominent sulcus. 
The large size of M. globosa restricts its comparison to a few species only. It 
differs from M. biconvexa in its more quadrate form and less inflated pedicle 
valve. It differs from M. sulcata in its lack of brachial folding and less inflated 
pedicle valve. 
The species as here conceived includes a number of specimens that seem 
clearly to be obese forms that thickened and widened but did not extend the 
hinge normally. Some of these occur with more normal members of the popula- 
tion, but one locality near New Bethel Church, Hilton (T.V.A. 197-NW) Quad- 
rangle, produce a majority of obese forms. The ornamentation of the thickened 
specimens is like that of the normal shells. For these reasons the specimens are, 
referred to M. globosa. 
It is important to state why the writer as first reviser of these species has 
used the specific name M. globosa which has page priority and is based on an 
adult specimen with pedicle interior. The species Pionodema globosa Willard, 
which proved not to belong to the genus to which it was originally assigned, was 
selected by Cooper (1930) as the type of the genus Mimella. It will there- 
fore cause little confusion to place H. melonica as a synonym of M. globosa rather 
than the other way around. Mimella superba of Butts is also placed in synonymy 
with M. globosa. The type specimen of that species externally is like M. melonica 
but is somewhat more strongly ribbed. It is not, however, more strongly ribbed 
than variants of the population which often lived together. 
MIMELLA GLOBULARIS Cooper, new species 
Plate 87, E, figures 25-37 
Shell of about medium size for the genus, wider than long; subrectangular 
in outline; strongly biconvex with the brachial valve having a slightly greater 
depth than the pedicle one. Anterior commissure broadly uniplicate ; sides some- 
what narrowly rounded. Hinge narrower than the greatest shell width; cardinal 
extremities obtusely angular. Costellate; costellae subequal in size, crowded 
and separated by striae of less width than the costellae. About 17 costellae in 
the space of 5 mm. at the front. 
Pedicle valve moderately convex in lateral profile with the greatest convexity 
located in the posterior half, the anterior half somewhat flattened. Anterior pro- 
file broadly convex. Sulcus nearly obsolete, the only trace shown is a broad 
flattening in the anteromedian third of the valve. Posterolateral slopes very 
steep but becoming more gentle anteriorly. Anterolateral and anterior slopes 
gentle. Beak erect and prominent, only slightly incurved; interarea long, 
apsacline. 
Brachial valve strongly convex in lateral profile; broadly convex in anterior 
profile. Umbo marked by a shallow and narrow depression that extends from 
the beak nearly to the middle of the valve. Fold obsolete, but a broad wave of 
