810 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 127 
the fold is narrowly rounded and conspicuous, often causing the anterior margin 
to become somewhat nasute. In one specimen 2 narrow folds were produced, 
but this is quite unusual. 
Eoplectodonta alternata is not likely to be confused with any other known 
species in lower Middle Ordovician rocks. Its large size, tumid valves, rounded 
sides, strong wrinkling, and the good development of denticles all distinguish 
it from E. ? dubia. The form and convexity of the valves differentiate this 
species from E. foerstei. 
EOPLECTODONTA ? DUBIA Cooper, new species 
Plate 207, A, figure 1; plate 209, C, figures 8-13 
Shell of about the usual size for the genus, wider than long, with the width 
at the hinge not quite twice the length. Cardinal extremities acute, often alate. 
Anterior margin broadly rounded; lateral margin variable. Surface marked by 
costellae of unequal size; young shells marked by about 11 strong costellae sepa- 
rated by about I0 fine, crowded lines. In full-grown specimens the sectors de- 
fined by the primary strong costellae are divided by costellae of intermediate 
size, and these are separated from the larger costellae by 4 to Io of the fine cos- 
tellae. About 7 costellae occupy a space of I mm. at the front margin. Posterior 
margin wrinkled. 
Pedicle valve gently convex in lateral profile with the greatest convexity in 
the posterior portion of the valve. Center part swollen to form a low fold. Flanks 
slightly convex, descending to the cardinal extremities with a gentle slope. Inter- 
area apsacline. 
Brachial valve with the greatest concavity in the umbonal and median regions, 
somewhat flattened in the vicinity of the cardinal extremities and anterolateral 
region. Interarea hypercline. 
Interior: Pedicle valve with adjustor-diductor impressions short, deeply im- 
pressed, somewhat elliptical in outline. From the anterior end of this muscle 
field a strong pallial trunk is sent out anterolaterally. Dental plates short, promi- 
nent, not hidden by callus. Accessory dental arrangement obscure, if present. 
Inside the brachial valve 3 septa extend to about the middle of the valve; the 
median septum is the highest and best developed of the 3. Interior of both valves 
strongly granulose. No hinge denticulations seen. 
Measurements in mm.— 
Hinge 
Length Midwidth width Thickness 
Holotype (pedicle valve) ........s.e008 10.1 16.3 18.9 ? 
Paratype ( “ 2 ap7ssha )vie eine 9.3 14.4 18? ? 
" CLeget: oe PRIZE SSD) po euctersrew’s 8.3 13.7 16.2 ? 
. a al hi jk Le SE Nase cleaieals 8.0 14.1 ? ? 
i (brachial valve 117555e)....... 6.7 10.1 13.0 ie 
~ phen ee ELT SSSI cee 7.8 10.9 13.5 ? 
Types.—Holotype: 117555d; figured paratypes: 110984, 117555b,f-h; un- 
figured paratypes: 117555a,¢,e. 
