PART I CHAZYAN AND RELATED BRACHIOPODS—COOPER 891 
concavo-convex ; surface multicostellate, costellae unequal in size and crossed by 
strong concentric fila. Pseudopunctae forming a dense mat. 
Interior of pedicle valve with wide and shallow delthyrial cavity ; dental plates 
rudimentary to obsolete; muscle field large, subcircular, not surrounded by a 
thickened ridge; diductors flabellate; adductors small, associated with 2 ridges 
along the inner edges of the diductor scars; median ridge poorly developed. 
Pseudodeltidium short; chilidium fully developed. Inner margin usually with 
thickened rim. 
Interior of brachial valve with moderately thickened notothyrial platform; 
brachiophores short, flat blades partially buried in the adventitious shell material 
of the socket and notothyrial platform. Median ridge short and low; adductor 
field marked by an oblique, low ridge; 2 lateral ridges and 1 median ridge ap- 
pear anterior to the broad median ridge attached to the notothyrial platform. 
Inside the margin marked by a subperipheral thickening. 
Genotype.—Macrocoelia obesa Cooper, new species. 
Discussion—This is a genus including species of large-sized shells, mostly 
with broadly convex profiles and without marked geniculation. Distinction be- 
tween this genus and some forms of Opikina is difficult. The shell structure of 
Macrocoelia is densely pseudopunctate like that of Opikina and thus quite differ- 
ent from Rafinesquina to which many of these shells were hitherto referred. 
In the pedicle valve the delthyrium of Macrocoelia may be unmodified by a 
pseudodeltidium or only partially covered by a remnant of a pseudodeltidium. 
In Opikina on the other hand the pseudodeltidium is usually well developed. The 
musculature of the two genera is very similar, but that of Macrocoelia may be 
somewhat larger. A difference exists between the two genera in the peripheral 
rim, that of Macrocoelia being farther toward the anterior than in Opikina. This 
is undoubtedly a manifestation of the lack of geniculation in Macrocoelia. 
The brachial valves of Macrocoelia and Opikina are similar in plan but differ- 
ent in detail. The adductor ridges are 4 in number in both genera, but those of 
Macrocoelia are not strong and in most of the species are poorly developed, if at 
all. The median ridge of Macrocoelia is strong, in Opikina it is weak and in old 
shells generally covered up. 
As here identified Macrocoeha ranges from the Crown Point formation of the 
Chazyan group to the Benbolt formation. It is not an abundant fossil in any of 
the formations of the Appalachians or elsewhere. 
MACROCOELIA BELLA Cooper, new species 
Plate 234, B, figures 6-13 
Large, wider than long, with the hinge slightly narrower than the width at 
the middle; sides gently rounded, subparallel to sloping gently inward; cardinal 
extremities nearly a right angle; surface costellate, costellae variable. Ornamen- 
tation in posterior and median portions of brachial valve consisting of low cos- 
tellae of 2 sizes, the larger ones separated by 1 to 8 smaller ones and the cos- 
tellae becoming more even in size anteriorly, numbering 4 to the millimeter at 
the front margin. Costellae of pedicle valve less strongly differentiated and the 
