362 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 127 
the pedicle valve having a slightly greater depth. Anterior commissure recti- 
marginate to faintly sulcate. Surface marked by high, narrowly rounded to angu- 
lar costellae, 6 in the space of 5 mm. at the front of a large valve. Imbrications 
distant, about Io in 5 mm. 
Pedicle valve moderately convex in lateral profile with the greatest convexity 
just anterior to the flattened umbo. Anterior profile moderately convex. Mid- 
region moderately inflated. Slopes to lateral margins fairly steep and flat; an- 
terior slope long and steep. Interarea moderately long, only slightly curved, 
strongly apsacline. Beak prominent, slightly incurved. Teeth small, dental plates 
short, receding. Muscle field large, extending to middle of valve. 
Brachial valve moderately convex in lateral profile with the greatest convexity 
in the midregion. Anterior profile gently convex. Sulcus shallow, occupying a 
little more than one-third the width at the front margin. Sulcus occupied by 2 
costellae at the umbo which increase by intercalation to 6 or more at the anterior 
margin. Second generation costellae intercalated on the inside of the primary 2; 
next generation intercalated on the outside of the primary 2. Umbo gently con- 
vex; interarea long, a little curved; beak only slightly incurved. Notothyrial 
cavity small and shallow; brachiophores short; cardinal process thin; median 
ridge low, extending anterior to middle. 
Measurements in mm.— 
Brachial Hinge 
Length length Width width Thickness 
Holotype i eters Pasth ie AY 13.7 12.5 I52 14.1 8.2 
Paratype |GP100728 )y o./.'u'ais «send Sle ee 11.7 10.8 12.6 9.6 7.6 
= (pedicle valve 1100I2c)... 13.8 ? 14.2 10.2 4.3 
ss (Ce Oe Ser aT TOOL) to. L985 ? 14.0 11.6 ? 
a (brachial valve 110013f) .._? 12.8 15.4 13.8 ? 
° CYWOTSay eos keees tose 5 11.0 13.9 11.8 ? 
Types.——Holotype: 110013j; figured paratypes: 110012c, 110013b-d,f,g, 
116928c,f-i; unfigured paratypes: 110012a,b,d-f, 110013a,e,h,i, 116928a,b,d,e. 
Horizon and locality—Ridley formation (lower third) in Tennessee: At Mur- 
freesboro ; east side Marshall Knobs, 5 miles south of Murfreesboro, Murfrees- 
boro (15’) Quadrangle. 
Discussion.—This species is characterized by the strong convexity of both 
valves, lack of perceptible fold or sulcus on the pedicle valve, and the fairly deep 
and prominent brachial sulcus. The latter feature is a variable one, but in gen- 
eral the sulcus is deepest between the two primary median ribs but may be defined 
on each side of them as well. 
Glyptorthis assimilis is most like G. bellatula from the Chatham Hill formation 
north of Marion, Va., but differs in having a more convex pedicle valve particu- 
larly in the median and posterior parts, in having a prominent pedicle umbo and 
beak, and a more strongly incurved beak. The pedicle muscle area of the Tennes- 
see species is in general somewhat larger than that of G. bellatula. The brachial 
valve of G. assimilis is less convex than that of G. bellatula, and the sulcus is 
less prominent. The ornamentation of the two species is different, that of G. as- 
