738 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 127 
folded portion of umbonal region short ; front half of valve gently concave ; lateral 
regions nearly flat ; extremities deflected slightly toward the brachial valve. Inter- 
area moderately long, apsacline. 
Brachial valve forming a gentle sigmoidal curve in lateral profile; umbonal 
region narrowly sulcate, sulcus shallow, noticeably extending to the middle of 
the valve and more indistinctly extending nearly to the front margin; anterior 
half gently swollen and folded in anterior profile to correspond to concave por- 
tion of the pedicle valve. Lateral areas flattened to gently concave; interarea 
hypercline, short. 
Interior of pedicle valve only slightly thickened in the young but with large 
subperipheral callosities and deeply intrenched pallial marks in old shells. In- 
terior of brachial valve with septa short and elevated to a crest at about the 
middle. Adductor impressions deep. 
Measurements in mm.— 
Brachial Hinge Thickness 
Length length Width width 
TA GIGI DE aie iameraie «oo hveibass,e's 00-018 8.4 7.2 12.0 13.3 2.4 
Paratype (pedicle valve 110923a)... 7.6 ? 10.7 14.0 ? 
5 i ae © 'TTOO2SD) 4.0, (8.0 ? 12.7 13.9 ? 
a Gir? Sah Groo2sh)ia..° 5:0 ? 8.8 9.6 ry 
rs (brachial valve 110923c).. 8.4 ? 10.2 11.5 r 
vs Geir’ ‘oh prog2gn ie. 7B ? 11.9 12.5 ? 
ey (08 “* 110923e).. 6.7 ? 12.0 14.7 ? 
¢ fo ae oy RTOOZO)...<'.. 78 7.0 11.7 13.0 2.1 
Types.—Holotype: 110918a; figured paratypes: 110923a-d,g; unfigured para- 
types: I110918b, 110920, 110923¢e,f. 
Horizon and locality—Base of Athens formation in Tennessee: Just east of 
intersection, 24 miles southeast of Riceville, Calhoun (T.V.A. 125-SW) Quad- 
rangle; 660 feet S. 40° E. of the railroad intersection 1 mile north of Athens, 
Athens (T.V.A. 125-SE) Quadrangle. 
Discussion.—Isophragma ricevillense attains a fairly large size for the genus 
and differs from J. biseptatum in size and thickness of shell, a lesser develop- 
ment of the median ridge, and more clearly defined pallial impressions. It differs 
from I. extensum in a lesser development of cardinal extremities and somewhat 
coarser and more even development of the costellae. 
ISOPHRAGMA SUBABBREVIATUM Cooper, new species 
Plate 169, A, figures 1-9 
Shell slightly wider than long, subrectangular in outline; valves thick. Cardi- 
nal extremities in the young acutely angular but forming nearly a right angle 
in the adult. Front margin subtruncate; anterolateral margins broadly rounded ; 
lateral margins oblique. Surface multicostellate, 7 costellae occupying a space 
of 2 mm. at the front of the holotype. Costellae nearly equal in size. 
Pedicle valve with a moderately strong but narrow fold extending from the 
beak nearly to the front margin. Front third of valve depressed into a broad, 
