970 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 127 
in the vicinity of the umbo; anterior profile broadly and moderately convex. 
Fold represented by a gentle longitudinal swelling from the umbo to the front 
margin. Lateral slopes flat and moderately steep. Interarea short and strongly 
incurved. Beak low, incurved. 
Brachial valve with the greatest convexity at the umbo in lateral profile; an- 
terior profile moderately convex but with a shallow depression in the medial 
region. Sulcus shallow and broad, originating at the umbo but becoming indis- 
tinct at the front of old shells. Posterior half of shell moderately swollen, an- 
terior half depressed. Flanks bounding sulcus gently swollen. Lateral slopes 
short, slightly concave and moderately inclined. 
Interior of pedicle valve with muscle field usual for the genus but much thick- 
ened at the front margin and with a thick callosity tapering anteriorly from the 
margin. Adductor track thickened and elevated. Brachial interior with unusually 
long brachiophores, cardinal process ranging from a thin septum to a thick boss, 
and moderately elevated median ridge. 
Measurements in mm.— 
Brachial Hinge 
Length length Width width Thickness 
Holotype) AoE. Fe 12.2 10.9 13.8 9.9 5.9 
Paratype (117286q)... 12.2 11.2 13.8 0.4? 6.5 
ii (@a720rb.) .ah.72'5 T.3 nee 9.6 78 
et (117291a)... 11.6+- 10.7 13.4 10.9 6.9 
ue (8r7202b) . Ait II.2 12.4 Q.1 6.0 
Types.—Holotype: 117287b; figured paratypes: 117286a-c,q, 117287a, 117288, 
117289a, 117291a,b, 117292a-c (d missing) ; unfigured paratypes: 117286d-p, 
117289b, 11729Ic. 
Horizon and locality—Lincolnshire formation in Tennessee: 4 mile north- 
west of Eidson, Kyles Ford (T.V.A. 170-SE) Quadrangle; in Clinch Valley 
at Shiloh Church near center northwest subquad., Pressmens Home (T.V.A. 
171-NE) Quadrangle. 
Same formation in Virginia: Along Tumbling Run, 14 miles southwest of 
Strasburg, Strasburg (15’) Quadrangle. 
Lincolnshire formation (Hogskin member) in Tennessee: Sally Cleveland 
Farm # mile southwest of Washburn, Dutch Valley (T.V.A. 154-SE) Quad- 
rangle; 0.4 mile east-northeast of Red Hill, 4 miles west-southwest of Thorn 
Hill, Avondale (T.V.A. 162-SW) Quadrangle; road to Washburn 3 miles west- 
southwest of Thorn Hill, Avondale (T.V.A. 162-SW) Quadrangle; $ mile north 
and northwest of Eidson, Kyles Ford (T.V.A. 170-SE) Quadrangle. 
Discussion.—This species attains an unusually large size for the genus but is 
not the largest of the species known in the Appalachians. Its shape is somewhat 
variable, but in general it is not so circular as P. ponderosa. The latter form is 
a much thicker shelled species and attains a much larger size. The fascicles of 
the Lincolnshire species are smaller and less prominent than those of P. pon- 
derosa. Young specimens of P. magna are suggestive of P. catawbensis but have 
rounded sides and a more broadly rounded anterior margin. 
The interior of P. magna is also quite distinctive. The adductor track in the 
