708 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 127 
oblique ; posterior margin nearly straight. Shell multicostellate with about 11 of 
the primary and intermediate costellae in 1 mm. at the front of a large adult. In 
the younger stages the surface is marked by numerous primary and intermediate 
costellae. Very fine costellae occupy the spaces between the larger costellae. 
Obscure oblique wrinkles occur just anterior to the posterior margin. 
Pedicle valve nearly flat in lateral profile, the posterior third to half very 
slightly convex and the anterior portion very gently concave. About the pos- 
terior half of the valve representing the shell up to the stage when it had reached 
5 mm. in length is slightly swollen in the umbonal and median region. Anteriorly 
and anterolaterally of this convex stage the valve is very gently concave. The 
front portion is usually flattened, but the anterolateral portions are generally de- 
flected slightly in the direction of the pedicle valve. Interarea nearly flat, strongly 
apsacline. Deltidium long and narrow. 
Brachial valve moderately to deeply concave in the median and umbonal por- 
tions ; concavity extending to about or slightly anterior to the middle. Postero- 
lateral areas just anterior to the posterior margin may be flattened or gently 
concave. Lateral and anterolateral areas gently convex with gentle slopes to the 
margins. Anterior slope gentle. Cardinal angles flattened to moderately convex 
with moderate slopes to the margin. Interarea short, anacline; chilidium short, 
strongly arched. 
Interior of pedicle valve with stout double teeth, no dental plates, wide and 
moderately long muscle field; diductor scars elongate and with thickened cal- 
losity at their anterior ends; pallial marks strong. 
Brachial valve having moderately elevated septum highest at its anterior end 
and extending for about two-thirds the length. Brachiophores short and stout. 
Cardinal process a simple ridge on a stout but not greatly elevated shaft. Second- 
ary ridges from the chilidium are located on each side of the median cardinal 
process. Pallial sinuses deeply impressed. 
Measurements in mm.— 
Brachial Hinge 
Length length Midwidth width Thickness 
1 a) 3 (017 Ce ER Cg eRe ee 11.9 TY 15.7 21.6 i 
Paratype’ (1110051) SPs. 9.3 8.9 15.1 19.0+ 1.8 
Types.—Holotype: 111065g; figured paratypes: 111065b,c,e,f,h-j; unfigured 
paratypes: 111065a,d. 
Horizon and locality—Arline formation in Virginia: At the entrance to 
Porterfield Quarry, 5 miles east of Saltville, Maccrady (T.V.A. 218-NW) 
Quadrangle. 
Discussion.—This species is characterized by its wide hinge and mucronate 
cardinal extremities. In this respect it is most like P. angulata from which it 
differs in its much greater size and different proportions. Inside the pedicle valve 
the muscle scars of P. angulata are more clearly defined, the teeth are smaller, 
and there is a lesser development of the pallial marks. 
