314 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 127 
Types.—Holotype: 116878b; figured paratype: 116878a,d, 116880, 116881; 
unfigured paratype: 116878c. 
Horizon and locality——Arline formation in Tennessee: 100 yards southwest 
of the Negro Cemetery, 4 mile northeast of Friendsville, Concord (T.V.A. 
138-SW ) Quadrangle. 
Little Oak formation in Alabama: On U. S. Highway 31, 4 mile north of 
the bridge, Pelham, Bessemer Iron District (15’) Quadrangle. 
Botetourt formation in Virginia: On Virginia State Highway 114, 2.3 miles 
west of the Montgomery-Roanoke County line, 12 miles northeast of Blacksburg. 
Arline formation in Virginia: Quarry just north of Marion, Marion (T.V.A. 
218-SE) Quadrangle. 
Discussion —This species is very similar to Orthambonites rotundifornus but 
is more strongly costate and the costae are more angular. O. tennesseensis is a 
much more robust shell than O. rotundiformus. 
ORTHAMBONITES TENUICOSTATUS Cooper, new species 
Plate 38, E, figures 30-40 
Shell small for the genus, wider than long, subrectangular in outline; hinge 
narrower than the greatest shell width which is at the middle; sides nearly 
straight or very gently rounded; anterior margin broadly rounded; surface 
marked by 34 to 40 elevated and narrowly rounded costae separated by spaces 
equal to or slightly wider than the costae ; costae appearing in 2 generations, one 
at the beak and the other on the anterior slope of the umbo. 
Pedicle valve strongly convex in lateral profile; greatest depth at about the 
middle; anterior profile swollen in the middle and with moderately long and 
gently convex, fairly steep, lateral slopes. Umbo swollen and beak incurved; 
median region inflated to form an inconspicuous fold; anterior slope steep ; lateral 
slopes gently inflated; umbonal slopes steep. Interarea moderately long, apsac- 
line. Interior with small teeth, short, thick dental plates; umbonal cavities filled 
with shell material ; delthyrial cavity deep; floor of muscle area slightly thickened 
anteriorly ; vascula media and median ridge not developed. 
Brachial valve gently convex in lateral profile; greatest depth located in the 
posterior third ; anterior profile broadly convex and gently indented in the median 
area; sulcus originating at the umbo, widening to the anterior margin where it 
occupies slightly less than 2 mm. Flanks bounding sulcus gently inflated ; umbonal 
slopes short and fairly steep; posterolateral extremities somewhat flattened. In- 
terior with narrow, shallow notothyrial cavity, but thickened notothyrial plat- 
form; cardinal process a low ridge, not well developed ; median ridge poorly de- 
fined, extending to about the middle; brachial process short and stout. 
Measurements in mm.— 
Brachial Hinge 
Length length Width width Thickness 
PTO BRI i... ss alabe ls we. ddweaishew tices 3.5 3.3 4.3 a7, 1.8 
Paratype (pedicle valve 116875h).... 6.5 ? 8.1 7.0 2.5 
pS (brachial valve 116875f)... ? 5.4 6.8 6.2 1.2 
