808 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 127 
EOPLECTODONTA ALTERNATA (Butts) 
Plate 184, A, figures 1-3; plate 207, B, figures 2-25; plate 208, A, figures 1-18; 
plate 269, E, figures 23, 24 
Sowerbyella alternata Butts, Virginia Geol. Surv. Bull. 52, pt. 2, p. 100, pl. 95, figs. 22, 23, 
1942. 
eae sp., ibid., p. 109, pl. 95, figs. 20, 21. 
Shell large, wider than long, length about two-thirds the width. Cardinal 
extremities acute or rounded and obtuse, depending on age. Lateral margins 
nearly straight or moderately rounded. Anterior margin broadly rounded to 
nasute. Anterior commissure sulcate. Hinge forming widest portion of shell in 
young but narrower than width at middle in old valves. Surface multicostellate, 
costellae of unequal size, about 30 of the larger ones at the front margin sepa- 
rated by from 3 to Io of the finer ones. The latter costellae are low, rounded, 
crowded, and with interspaces much narrower than the costellae. Larger cos- 
tellae rounded and elevated. Fine concentric fila and wrinkles along the pos- 
terior margin complete the ornamentation; as many as 18 wrinkles occurring 
on each side of the beak. 
Pedicle valve with low beak, protruding very slightly posterior to the posterior 
margin. Umbo slightly swollen, extended forward as a low, narrow fold. An- 
terior profile narrowly arched with the greatest height at the narrow fold and 
the gently convex sides sloping steeply to the margins. Lateral profile with the 
greatest convexity located in the umbonal region. Cardinal extremities deflected 
toward the pedicle valve to form small ears. Interarea curved, orthocline. 
Pseudodeltidium strongly arched, fairly large. 
Brachial valve deeply concave with the deepest part in the posterior half; not 
strongly sulcate. Cardinal extremities deflected toward the pedicle valve; inter- 
area nearly flat and usually nearly in a plane with the interarea of the pedicle 
valve and therefore nearly orthocline in position. 
Pedicle valve with muscle field short, extending about one-quarter the length; 
adjustor-diductor marks elliptical in outline, widely divergent, separated by a 
short, stout median ridge. Adductor impressions forming small pits. Teeth 
small ; hinge denticles about 6 or 7 in number. Entire surface papillose. Brachial 
valve with septa extending to about the middle where they are moderately ele- 
vated. Median septum stronger than the lateral septa. Adductor impressions 
indistinct. 
Measurements in mm.— 
Brachial Hinge 
Length length Midwidth width Thickness Height 
Hectotype’ ne Bae. 2% 11.0 ? 15.6? ? ? 4.4 
Hypotype (79419a)...... 12.7 ? 17.8 17.2 ? 5.0 
: C7QATOD) si 0's « 13.8 ? 19.0 20.8 ? 6.7 
i (110993d)..... 12.0 12.0 20.3 21.6 ? ? 
" (117551-1) .... 16.1 ? 20.7 19.6 ? 8.0 
aS 127552) eae: 13.8 II.0 19.3 20.2 ? 4.9 
ve (117550b)..... 139 r7.2 18.2 18.0 a1 6.1 
a (2T7VES0G) .. . 4s 13.3 Ter 18.2 16.0 Eile 5.7 
- CLI75S50d) . 0s 13.1 II.4 ‘19.0 18.0 2, 5.5 
