PART I CHAZYAN AND RELATED BRACHIOPODS—COOPER 319 
has yielded many. In general the genus can be recognized by its concavo-convex 
outline and its usually fine costae or strong costellae. The outline is thus simi- 
lar to that of Orthis ss. to which the genus is most closely related. The pedicle 
interior is characterized by short and flaring dental plates and a muscle field 
rhomboidal in outline. Inside the brachial valve the cardinalia are distinctive. 
The brachiophores are characteristically orthoid but are very thin and incline 
laterally from the notothyrial margin at a very low angle. They bound narrow 
slitlike sockets. The cardinal process has a short shaft, and the myophore 
may be more or less crenulated. The median ridge is short and the adductor 
field small. 
Cyrtonotella is commonest in the lower part of the Middle Ordovician. Speci- 
mens in the collection, but too poor for identification or description, were taken 
from the Meadow Marble and “Ottosee” in the neighborhood of Knoxville, Tenn. 
It is also fairly common in the Tellico iron ore and limestones, but no specimens 
in the collection were good enough for description or identification. The genus 
ranges as high as the Benbolt but has not yet been seen above. Although Cyrto- 
notella is fairly common in parts of the Appalachians, it is known from western 
United States only from the shales between the Eureka sandstone and the upper 
Pogonip, where it is rare. 
In this monograph the genus Planidorsa Schuchert and Cooper is placed in 
synonymy with Cyrtonotella. Opportunity to examine good brachial interiors of 
Planidorsa shows that genus to have the characteristics of Cyrtonotella. The 
pedicle interior of the genera is likewise very close. Schuchert and Cooper are 
mistaken in their comparison of the pedicle musculature of Planidorsa to Multi- 
costella. The musculature is actually almost identical to Cyrtonotella. Species 
assigned previously to Planidorsa are usually more finely costellate than typical 
Cyrtonotella, and the brachial valve is often less concave, but these features do 
not at present seem sufficiently important for maintaining the name Planidorsa. 
In absence of significant interior differences between the two genera it seems 
best to submerge Planidorsa. At best, under modern methods it could be used 
only as a subgenus. 
CYRTONOTELLA BELLA (Schuchert and Cooper) 
Plate 41, E, figures 20-25 
Planidorsa bella ScHUCHERT and Cooper, Mem, Peabody Mus. Nat. Hist., vol. 4, pt. 1, p. 101, 
pl. 16, figs. 24, 25, 1932. 
Shell fairly large, planoconvex to gently concavo-convex in lateral profile, 
wider than long with the hinge forming the widest part. Cardinal extremities 
varying from nearly a right angle to alate. Lateral margins sloping medially; 
anterior margin broadly rounded. Anterior commissure rectimarginate. Sur- 
face multicostellate, costellae elevated and very narrowly rounded and sepa- 
rated by interspaces about equal in width to the width of the costellae or 
slightly wider. Costellae increasing by implantation and bifurcation in 4 gen- 
erations. About 7 or 8 costellae in a space 5 mm. at the front margin of an adult. 
