468 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 127 
Figured specimens.—118004a,b. 
Horizon and locality—Pierce formation in Tennessee: At Pierce Mill, on 
Tennessee Highway 10, just south of Walterhill, 74 miles north-northeast of 
Murfreesboro, Rutherford County. 
Genus MIMELLA Cooper, 1930 
Mimella Cooper, Journ. Paleont., vol. 4, No. 4, p. 375, 1930.—SCHUCHERT and Cooper, Mem. 
Peabody Mus. Nat. Hist., vol. 4, pt. 1, p. 61, 1932. 
Mimella can be recognized by its generally hebertelloid outline, but its pedicle 
valve is usually more convex than that of Hebertella. The pedicle muscle scar 
of Mimella is the most important and diagnostic feature of the genus. This is 
characterized by subparallel or divergent, elongate and deeply sunk diductor 
tracks separated by an elongate and usually anteriorly expanded and elevated 
adductor track. The brachial interior is like that of the rest of the hebertelloids. 
Small distinctions are possible between Marmor and post-Marmor Mimellas. 
The earliest forms generally are rectimarginate, but they also have a fairly 
strongly pronounced sulcus in the umbonal and median regions. Generally the 
sulcus does not reach the anterior margin. Post-Marmor Mimellas are often 
umbonally sulcate, but the anterior commissure is usually broadly uniplicate. 
The folding of the anterior commissure is usually confined to the marginal region 
of the valves because no species is known with a strong and pronounced median 
fold on the brachial valve. These features are of some help in sorting the species 
stratigraphically. 
A note on the genotype of the species is important. When the genus was 
established, Cooper cited Pionodema globosa Willard as the type species. Willard 
also described Hebertella melonica which belongs to the genus Mimella. The 
results of the present studies indicate that these two species are the same. It 
seems best, therefore, to put H. melonica in the synonymy of M. globosa be- 
cause the latter species was selected as the type of the genus. 
Mimella, according to present studies, appears first in the middle of the 
Chazyan (Crown Point) and extends into the Wardell formation. The genus 
is abundant in the Benbolt formation where it attains a large size. Doleroides 
appears first in the Dryden formation and becomes fairly common in Wardell 
rocks. In still higher rocks of the lower Trentonian, Doleroides is the abundant 
hebertelloid. 
MIMELLA BICONVEXA Cooper, new species 
Plate 87, C, figures 16-19 
Large, wider than long, transversely elliptical in outline; hinge narrower than 
the greatest width which is at about the middle. Cardinal extremities obtuse. 
Anterior commissure gently uniplicate. Surface covered by nearly uniform, fine 
costellae about 14 in 5 mm. at the front. Costellae exopunctate. 
Pedicle valve with lateral profile fairly evenly convex with the maximum con- 
vexity near the middle. Sulcus originating at about the middle, narrow and 
