494 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 127 
Horizon and locality—Edinburg formation (lower Liberty Hall facies) in 
Virginia: 1.8 miles S. 4° W. of Bethel Church, 5.9 miles east of Harrisonburg, 
Harrisonburg (15’) Quadrangle: 4 miles southwest of Bland. 
Red Knobs formation in Tennessee: 6 miles east of Knoxville. 
Discussion.—This species is characterized by the strength of its ornamenta- 
tion, the strong median costa which forms a prominent fold, late intercalation of 
the secondary generation of costae, the deep and wide sulcus in the brachial 
valve, and the flatness of the same valve. 
As the ornamentation is the most distinctive feature of this little shell, a few 
more remarks are necessary. A specimen 1.5 mm. in length may show only pri- 
mary costellae, but two specimens actually somewhat shorter show intercalation 
of the secondaries on each side of the median costa, and occasional costae else- 
where on the front margin. The median costa maintains its individuality although 
the secondary costae intercalated on each side of it are also slightly elevated. 
The interiors of both valves of this species are characterized by the robust 
development of septa and spondylium. The latter is short and shallow and sup- 
ported by a considerable thickening at the posterior. 
The outline and general form of S. rectangulatus are like those of S. costatus, 
but the former is less strongly costate and has a pedicle valve with more gentle 
lateral profile. The Benbolt species, S. mediocostatus, is fairly strongly costate, 
but it does not have as strong a fold and sulcus as S. costatus. 
SKENIDIOIDES ELONGATUS Cooper, new species 
Plate 92, C, figures 10-22; plate 98, A, figures 1-5 
Large for the genus, length and width approximately equal ; in some specimens 
the width is the greater dimension, in others it is the length. Outline sub- 
pentagonal ; sides rounded, sloping obliquely toward the middle; anterior margin 
somewhat narrowly rounded. Anterior commissure deeply sulcate. Surface 
costate, costae numbering 28 to 32, low and rounded and with intercostal spaces 
narrower than the costae. Costae mostly direct from beak to anterior margin, but 
intercalation takes place along the margin of large specimens. Fold with a pri- 
mary bifurcation at the beak and 3 sets of intercalated costae anteriorly to pro- 
duce 8 costae. Corresponding ribbing in sulcus. 
Pedicle valve evenly and gently convex in lateral profile; anterior profile sub- 
carinate ; beak often distorted; umbo narrowly swollen; fold narrowly convex; 
flanks flattened and steep to the margins. Interarea unusually long, strongly 
apsacline, curved and occasionally distorted. 
Brachial valve gently convex in lateral profile with the anterior bent fairly 
strongly toward the pedicle valve; anterior profile broadly and gently convex but 
with the median region sulcate. Sulcus originating about 4 mm. anterior to the 
beak, widening and deepening anteriorly to occupy nearly half the width at the 
front margin. Tongue moderately long and rounded. Flanks bounding sulcus 
narrowly rounded ; posterolateral areas flattened to gently concave. 
