PART I CHAZYAN AND RELATED BRACHIOPODS—COOPER 867 
cardinal extremities subrectangular to subauriculate; convexi-concave in the 
adult but concavo-convex when young; anterior commissure rectimarginate. 
Surface finely costellate, strong and fine costellae alternate and appear in sev- 
eral generations; entire surface covered by fine concentric fila. Pseudopunctae 
finely and densely matted. 
Pedicle valve with minute pedicle foramen, pseudodeltidium of moderate size 
and convexity; teeth small, surface smooth or roughened, supported by callus 
thickening; muscle area large, occupying about half the valve area; diductor 
scars broad and flabellate ; adductor scars small, located at about the middle of 
the muscle field on 2 low, subparallel ridges that extend anteriorly to about the 
valve middle where the diductors end. Posterior of delthyrial cavity filled by 
adventitious shell. Anterior and lateral margins with thickened inner rim. 
Brachial interior with thickened notothyrial platform bearing 2 stout lobes of 
the cardinal process having short shafts and flat myophore surfaces. Brachio- 
phores visible as short, flat blades on the inner side of the sockets and nearly 
buried by adventitious shell material. Median ridge short and low ; adductor field 
marked by 4 short radiating ridges, 2 on each side of the median ridge; moder- 
ately thickened rim just inside margins, but not so prominent as that of the 
pedicle valve. 
Genotype.—Strophomena tennesseensis Willard, Bull. Mus. Comp. Zool., 
Harvard Coll., vol. 68, No. 6, p. 285, pl. 2, figs. 17, 18, 1928. 
Discussion.—This genus is characterized by gentle geniculation of the valves, 
quadrate outline, and with length and width nearly equal. Most of the species 
have the sides nearly parallel. Inside the pedicle valve the muscle field is very 
large and flabellate, often occupying nearly half the area of the interior. The 
diductors are commonly separated by 2 parallel ridges that extend from the 
posterior to the anterior end of the muscle scar. The interior is also marked by 
a prominent ridge inside the margin. Dental plates are absent, and no marked 
thickening is laid under the dental ridges which simulates a dental plate as in 
Strophomena. 
The brachial valve is characterized by small and delicate cardinalia for such 
large shells. The median ridge is very short, and the socket cups of adventitious 
shell formed around the brachiophore are small. Two short ridges may appear 
in connection with the adductors on each side of the median ridge. 
Rhipidomena is an early strophomenoid which differs from Strophomena in 
its external habit, having a quadrate form and modest geniculation and differing 
internally in the form of the pedicle muscle region and the thick inner, sub- 
peripheral rim. In Strophomena a ridge of adventitious shell generally nearly 
surrounds the muscle scars, and the muscle field is proportionately smaller than 
that of Rhipidomena. The brachial interiors of both genera are basically the 
same. 
RHIPIDOMENA FILICOSTELLATA Cooper, new species 
Plate 254, B, figures 4-6 
Shell of about usual size for the genus, subquadrate in outline and with the 
length and width nearly equal ; hinge narrower than the greatest shell width which 
