PART I CHAZYAN AND RELATED BRACHIOPODS—COOPER 733 
Subfamily IsopHRAGMINAE Cooper, new subfamily 
Small, resupinate, costellate shells having an exterior like Palaeostrophomena 
but with 2 long, parallel septa in the brachial valve. 
ISOPHRAGMA Cooper, new genus 
(Greek isos, equal; phragma, fence) 
Shell small, usually wider than long with the hinge equal to or forming the 
greatest shell width. Resupinate; anterior commissure uniplicate ; brachial valve 
sulcate in umbonal region, usually plicate in anterior half. Surface multicostel- 
late ; pseudopunctate. 
Pedicle interior with small, stout teeth, having deep fossettes on their inner 
face ; dental plates short and stout, usually obsolete because of filling of umbonal 
cavities by adventitious shell substance. Delthyrial cavity moderately deep; del- 
tidium narrow and short. Sides of delthyrial cavity with marginal thickenings ; 
interarea long, apsacline. Muscle field with elongate and narrow diductor impres- 
sions and small obscure adjustor impressions. Adductor track wide. Pallial 
trunks strongly marked, major trunks extending anteriorly from diductor impres- 
sions, one branch extending anteromedially, and another laterally. A second 
major trunk is given off laterally from near the ends of the dental plates and ex- 
tends by a narrow curve anterolaterally. Several large callosities are located just 
within the anterior and lateral margins. 
Brachial interior with short brachiophores and much thickened notothyrial 
platform. Sockets deep, brachiophores moderately large, flattened plates sup- 
ported by lateral thickenings of the notothyrial platform. Cardinal process large, 
simple, situated on the posterior face of the notothyrial platform and in old shells 
produced forward as a ridge along the surface of the platform. Myophore car- 
inate. Anterior to the notothyrial platform 2 subparallel ridges extend anteriorly 
nearly to the front margin. These ridges rise to a crest at a point slightly anterior 
to or nearly at the center of the valve. Ridges often greatly thickened and the 
space between them filled by excess shell substance. Adductor impressions lo- 
cated on each side of the median ridges ; the posterior adductor scars the larger. 
Pallial impressions strong; 2 main trunks originating at the anteromedian ex- 
_ tremities of the anterior pair of adductor impressions and extending anteriorly, 
bifurcating near the margin; a second pair of trunks extending anterolaterally 
from the line of contact of the anterior and posterior adductor scars; a third pair 
extending laterally from the posterior ends of the posterior adductor scars. Inter- 
area hypercline. 
Genotype.—Isophragma ricevillense Cooper, new species. 
Discussion.—A casual study of these interesting shells suggests that the genus 
is a reversed Sowerbyella or Leptelloidea. Although the contour of the valves is 
reversed from that of the genera named, a sufficient number of internal structural 
differences between Isophragma and Sowerbyella and Leptelloidea are at once 
apparent and serve to distinguish the genera. Aside from its resupinate form, 
Isophragma is characterized by the strong subparallel lamellae forming the axis 
