726 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 127 
Lateral margins nearly straight to convex; anterolateral extremities strongly 
rounded. Surface marked by costellae of unequal size and narrow concentric 
wrinkles best defined between the hinge and the posterolateral region. 
Pedicle valve moderately convex with the greatest convexity in the anterior 
half where the valve is gently geniculated. Umbonal region gently convex with 
a gentle concave slope to the cardinal extremities. A low fold extends from beak 
to front margin but is inconspicuous on geniculated portion. Midregion and an- 
terior half of shell somewhat swollen. Cardinal extremities slightly deflected. 
Interarea long, apsacline ; pseudodeltidium rudimentary. 
Brachial valve gently concave in the posterior two-thirds but geniculated 
toward the brachial valve in the anterior third. Greatest concavity located just 
posterior to the geniculated part of the valve. Shallow median depression extend- 
ing from the beak to geniculated part. Lateral regions of valve gently concave, 
becoming less concave toward the cardinal extremities. Umbo concave. Inter- 
area short, hypercline. 
Interior of pedicle valve with large callosities in front of the diductor-adjustor 
fields, well-developed pallial sinuses and well-defined genital impressions. In- 
terior of brachial valve with high, thick median septum, large cardinal process, 
and thick but short lateral septa. 
Measurements in mm.— 
Brachial Hinge 
Length length Width width Thickness 
Higlotyne et vcicve! shee. zeenies x 13.0 11.5 22.7 25.0 28 
Paratype (pedicle valve 117401a)... 13.0 2 23.0 21.4 ? 
‘ (brachial valve 11740I1b)..  ? 14.5 26.4 23.3 ? 
Types.—Holotype: 117401z; figured paratypes: 117401b,d,e,h,k,m,n,p,q,v ; 
unfigured paratypes: 117401a,c,f,g,i,j,l,o,r-u,w-y. 
Horizon and locality.—Lincolnshire formation (8 feet below Cybeloides bed) 
in Virginia: Tumbling Run section, 14 miles southwest of Strasburg, Strasburg 
(15’) Quadrangle. 
Discussion—The specimens on which this species is based were etched from 
limestone and are quite coarsely silicified; consequently, some of the details are 
obscured. The pallial markings and septa of the interior, however, are well pre- 
served, but the finer granulation of the internal surface and the finer costellae of 
the exterior are not retained in detail. On the best-preserved exterior of a 
brachial valve it is possible to count 6 fine costellae between the stronger ones 
which are about I mm. apart about one-third the distance from the anterolateral 
margin. 
This species is closest to Sowerbyites triseptatus (Willard) but is larger and 
less convex in its mature stages. In old shells the front third to half of the 
pedicle valve is geniculated toward the brachial valve. The front quarter or 
third of the brachial valve is geniculated and thickened. Specimens of the 
pedicle valve of S. gildersleevei comparable in size to the type specimens of 
S. triseptatus are less convex than the latter but have more widely divergent and 
larger diductor-adjustor scars in the pedicle valve. Brachial valves of S. gilder- 
