734 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 127 
of the brachial valve. Inside the pedicle valve the musculature is quite orthoid. 
Isophragma differs from Sowerbyella in several important features. Inside 
the pedicle valve of Sowerbyella a short but prominent median septum located at 
the posterior of the valve divides the muscle field. Furthermore, the arrangement 
of the muscles is not the same as in Jsophragma. Inside the brachial valve the 
notothyrial platform of Isophragma is not excavated under the cardinal process 
as it is in Sowerbyella, and the brachial processes are somewhat longer in the 
latter genus. Again in Sowerbyella the arrangement of the adductor scars of the 
brachial valve is not so orthoid as in Jsophragma. 
Leptelloidea and Leptella differ from the genus under discussion in the pos- 
session of an elevated visceral disk which is completely absent in Isophragma. 
In the United States this genus has a moderately long stratigraphic range from 
the Arline formation of the Knoxville area into the Benbolt formation. Although 
this is a rather extensive stratigraphic range, no species is known between the 
intervals named. In Great Britain Leptella? pseudoretroflexa Reed from the 
Llandeilo (Balclatchie formation) belongs to this genus. 
ISOPHRAGMA BISEPTATUM Cooper, new species 
Plate 171, D, figures 20-29; plate 171, E, figures 30-32 
Large for the genus, wider than long, with the greatest width at the hinge. 
Cardinal extremities varying with age from alate to approximately a right angle. 
Anterior margin broadly to narrowly rounded; lateral margins obliquely straight 
to slightly concave. Surface of an adult marked by about 75 narrowly rounded 
costellae having interspaces about the same width as the costellae. Growth 
lamellae strong, costellae often produced into short, blunt spines at the growth 
lamellae. Costellae crossed by minute elevated fila, at least 15 to 20 to the 
millimeter. 
Pedicle valve nearly flat in lateral profile, umbo slightly swollen, the swelling 
extending forward as a low fold for one-fourth to one-third the length. Valve an- 
terior to the fold flattened or slightly depressed to form a shallow sulcus, in some 
specimens the swelling continuing as a fold to the front margin. Tip of beak 
smooth and convex. Anterior and anterolateral portions of the adult very gently 
convex and with the cardinal extremities deflected slightly toward the brachial 
valve. Beak extended slightly beyond the posterior margin. Interarea broad and 
long, strongly apsacline. Deltidium long and narrow. 
Brachial valve unevenly convex in lateral profile with the posterior third flat- 
tened or concave and the anterior two-thirds gently to moderately convex ; gently 
convex in anterior profile. Umbo concave, the concavity a shallow pit just an- 
terior to the posterior margin. Concavity extended anteriorly as a narrow and 
shallow sulcus for a short distance only or nearly to the front margin in young 
or middle-aged shells. Anteromedian region gently swollen; anterolateral areas 
gently convex with moderate slopes to the depressions defining the cardinal alae. 
Interarea shorter than the pedicle one, moderately long and hypercline in position. 
Interior of pedicle valve with muscle area about as long as wide, with square 
