PART I CHAZYAN AND RELATED BRACHIOPODS—-COOPER 7O1 
Several species of Appalachian brachiopods appear to conform to the details 
of the emended Palaeostrophomena as described and illustrated by Opik. The 
Esthonian shells, however, differ from the American ones in size, thickness of 
shell, and ornamentation. The Esthonian specimens are more wrinkled and are 
larger than any of the American species. The interior details of the American 
species conform to those of Palacostrophomena described and illustrated by Opik. 
Details of the interior of the pedicle valve of the Esthonian form, however, are 
not known, and this gap in knowledge leads to some doubt as to precise generic 
assignment. In spite of this gap in our knowledge the American shells are as- 
signed to the emended Palagostrophomena. The following remarks on generic 
characters are based on the five known American species. 
The lateral profile is resupinate, i.e., the young shell is generally concavo- 
convex but in the adult the convexity is reversed. Thus the young stages of 
Palaeostrophomena have normal convexity, but the anterior of the pedicle valve 
is more or less deeply concave and that of the brachial valve is more or less 
strongly convex. The ornamentation consists of costellae of unequal size, the 
stronger ones separating groups of finer ones. Strong concentric wrinkles are 
a common feature of the Esthonian shells, but in the American species these are 
generally confined to the posterior margins. Both interareas are moderately long ; 
the notothyrium and delthyrium are both narrow and are covered by strongly 
elevated and narrowly convex chilidium and pseudodeltidium. 
Inside the pedicle valve the teeth form one of the most prominent features be- 
cause they are double, consisting of the tooth and an accessory dental process. 
The tooth consists of a flat blade that grows obliquely forward from the apex. 
Normally the inner edge of the tooth track forms the margin of the delthyrium, 
but in Palaeostrophomena a deposit of shell substance is laid on the inner side 
of the tooth to protrude at its anterior end. In articulation the tooth rests on the 
outer or socket surface of the brachiophore and the accessory tooth rests on the 
inner surface of the brachiophore. The narrow space between the tooth and the 
accessory tooth receives the carinate edge of the brachiophore. The accessory 
tooth seems to be formed of adventitious deposit. 
Dental plates are lacking in the adult forms of the American shells and seem 
also to be lacking from the specimens figured by Opik. False dental plates, how- 
ever, composed of adventitious shell extend from the teeth for a short distance 
on each side of the muscle field forming a fairly high ridge in some instances. 
The muscle field of the pedicle valve of all the known species is large and oc- 
cupies a generous portion of the interior. The field is generally pentagonal in 
outline with the muscle scars converging to the apex and subparallel on the 
sides. The anterior is deeply emarginate. Adjustor scars are visible at the base 
of the accessory teeth or their extensions, the false dental plates. The diductor 
scars are elongate, narrow, and usually extend to the valve middle or anterior 
to it. In some specimens low callosities appear at the anterior ends of the diductor 
scars. The adductor field forms a short, flattish, somewhat rectangular scar, often 
with longitudinal striation at the apex. Its anterior end never reaches the anterior 
