398 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 127 
Discussion.—This species is characterized by its slender profile and trans- 
verse form. It is very close to D. willardi but differs in its more transverse out- 
line, more concave pedicle valve, and much less convex brachial valve. 
DINORTHIS TRANSVERSA Willard 
Plate 58, B, figures 4-10; plate 58,C, figures 11-27; plate 268, J, figures 45-47 
Dinorthis transversa WILLARD, Bull. Mus. Comp. Zool., Harvard Coll., vol. 68, No. 6, p. 271, 
pl. 2, figs. 1, 2, 6, 1928—Butts, Virginia Geol. Surv., Bull. 52, pt. 2, p. 83, pl. 85, figs. 
14-16, 1942. 
Dinorthis quadriplicata WILLARD, idem, p. 271, pl. 1, fig. 13; pl. 3, figs. 5, 6, 1928. 
Species variable, outline subrectangular, width greater than length, greatest 
width at about the middle. Cardinal extremities rounded, varying from nearly 
a right angle to obtuse. Lateral margins broadly rounded, generally sloping 
slightly toward the center ; front margin nearly straight: anterolateral extremities 
somewhat narrowly rounded. Anterior commissure broadly uniplicate. Sur- 
face multicostate ; median 4 to 6 costae generally stronger than the others and 
usually unbifurcated in young parts of the shell. Costae on the flanks usually 
bifurcated near their midlength, but on old or large specimens additional bifurca- 
tion takes place near the front margin creating 3 generations of costae. Some 
specimens show bifurcation of the larger median costae at the front. 
Pedicle valve in anterior profile gently to moderately concave; in lateral pro- 
file umbonal region more or less strongly convex, while the front two-thirds are 
moderately to strongly concave. Swollen umbonal region short; median strong 
costae often indistinctly define a poorly developed fold at least in the posterior 
half. Interarea short, nearly procline. Flanks varying from concave to flat or 
very gently convex. 
Brachial valve moderately convex in lateral profile with the greatest convexity 
at about the middle; anterior profile moderately to strongly rounded, often with 
a moderately strong median depression. Sulcus originating at the beak, moder- 
ately to strongly impressed and generally extending to the anterior margin where 
it usually is shallower than on parts posterior to the margin. Flanks bounding 
sulcus narrowly swollen. Posterolateral slopes generally steep. Interarea short, 
orthocline. 
Pedicle valve with muscle area slightly wider than long, tapering slightly pos- 
teriorly. Adductor field large, nearly circular. A slight elevation separates an- 
terior ends of diductor impressions. Median ridge of brachial valve not quite 
reaching center, cardinalia small. 
Measurements in mm.— Bdciaat Hinge 
Length length Width width Thickness 
EMEHLGEV IDOE Lave, 7a cin slaps eishalsiamecea lee 20.7 212 30.6 22.0 ? 
Hypotype (110332a) ......... 17.4 18.1 23.9 21.2 6.7 
Holotype (D. quadriplicata).. 19.7 ? 24.9 18.5 ? ? 
Hypotype (98038)........... 17.4 18.5 25.1 17.3 6.8 
A (TT OZ2O Vives tis slob fe 13.0 14.1 19.7 172 3.9 
: CU UOGIE) oa ane Siriaas 20.5 21.8 30.4 22.0 8.9 
od CITTOOA ye. sss 20.6 21.4 30.0 24.9 7.6 
