7309 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 127 
Pedicle valve most strongly convex in the anterior third where the valve is 
strongly geniculated toward the brachial valve. Anterior slope moderately long 
and steep. Posterior two-thirds slightly convex. Umbo slightly swollen; beak 
small, protruding slightly posterior to the posterior margin. Median region 
slightly folded, but the fold is lost where the valve is geniculated. Lateral slopes 
moderately steep; interarea long for the genus, slightly curved, approximately 
orthocline. Pseudodeltidium small. 
Brachial valve concave with the greatest concavity at the place of genicula- 
tion. Umbo concave; median sulcus shallow and poorly to well defined; areas 
bounding sulcus nearly flat or concave. Cardinal extremities sulcate. Interarea 
hypercline. Chilidium short. 
Interior of pedicle valve with elongate diductors terminating in low callosities 
located at about the middle of the valve. Interior of brachial valve with promi- 
nent median septum highest at about the middle of the valve, lateral septa well 
developed, the inner 2 the stronger. 
Measurements in mm.— 
Brachial Hinge 
Length length Midwidth width Thickness 
EIOIGEY OE ca sce eae cn sree ean 12.1 10.6 15.0 17.2+ 3.9 
Paratype (111797d).......... 10.3 8.6 15.2 17.74 3.2 
3 COSrS 7a) ss To 10.6 9.0 16.3 18.2 2.9 
rf (1174072) oe pices lescwe 10.5 9.2 13.8 16.1+ 2.4 
Types.—Holotype: 111797g; figured paratypes: 98187a-c, 111797a,d,f; un- 
figured paratypes: 111797b,c,e,h, 117407a-d. 
Horizon and locality—Lincolnshire formation in Virginia: In Marcem 
Quarry, 2 miles west of Gate City, Gate City (T.V.A. 188-NE) Quadrangle. 
Discussion—This species is distinguished by its medium size, strong con- 
vexity of the pedicle valve and consequent deep concavity of the brachial valve, 
and the somewhat narrowly rounded anterior margin. It is smaller and deeper 
than S. triseptatus, S. lamellosus, and S. gildersleevei. Its size is similar to that 
of S. delicatus, but it is much more convex. 
SOWERBYITES TRISEPTATUS (Willard) 
Plate 160, B, figures 10-12; plate 180, A, figures 1-13; plate 182, B, figures 12-29; 
plate 183, B, figures 17, 18; plate 200, B, figures 4-7 
Plectambonites triseptatus Wr1LLarp, Bull. Mus. Comp. Zool., Harvard Coll., vol. 68, No. 6, 
pl. 2, figs. 7, 8, 1928. 
Shell of about usual size for the genus, subsemicircular in outline with the 
width at the hinge nearly twice the length. Cardinal extremities acutely angular 
to auriculate. Lateral margins gently rounded, more or less sloping in an antero- 
median direction. Anterior margin moderately strongly rounded to only gently 
rounded. Surface marked by concentric undulations, scalloped on their edges. 
These undulations are formed by thin shell lamellae concentrated on the surface, 
and the scallops are produced by the undulation of the lamellae over costellae 
underneath. On the surface the costellae appear as depressed lines or radial 
