PART I CHAZYAN AND RELATED BRACHIOPODS—COOPER 329 
Discussion.—This species differs from T. emarginata by its finer costellation, 
and shallow sulcus becoming obsolete anteriorly. 
PLEURORTHIS Cooper, new genus 
(Greek pleuron, rib) 
Shells ranging from small to fairly large, generally subrectangular to semi- 
elliptical in outline ; biconvex, with the brachial valve generally the deeper ; hinge 
wide, straight, usually equal to or slightly less than the greatest shell width; shell 
sulcate in young stages but becoming uniplicate in the adult ; surface multicostel- 
late ; shell fibrous, impunctate. 
Pedicle valve with short receding dental plates, dental fossettes deep ; muscula- 
ture orthoid ; delthyrium open. Brachial valve with short median ridge. 
Genotype.—Pleurorthis fascicostellata Cooper, new species. 
Discussion.—This genus is characterized by its fairly typical orthoid characters 
combined with a peculiar ornamentation. The valves are biconvex, and the 
brachial valves are sulcate in young and early mature stages. The sulcation, how- 
ever, gives way to a more or less prominent plication. In adults and old shells 
the plication is broad and prominent. The fold is generally low but the sulcus 
in the pedicle valve is broad and deep. The great depth and breadth developed 
in some specimens early led to the assignment of some species of this genus to 
Sirophomena. Usually the multicostellate exterior combined with the convex 
valves and the broad uniplicate commissure will separate this genus from other 
orthoids. 
PLEURORTHIS CONVEXA Cooper, new species 
Plate 31, D, figures 15-21 
Shell small for the genus, subquadrate to subelliptical in outline ; lateral mar- 
gins narrowly rounded; anterior margin broadly convex; valves strongly con- 
vex ; surface multicostellate to fascicostellate, 3 or 4 costellae to the millimeter 
and bundled in groups of 3 or 4 at the front margin. 
Pedicle valve fairly strongly convex in lateral profile and with the greatest 
convexity at about the middle; anterior profile broadly convex and somewhat 
carinate medially ; fold narrow and prominent on the umbo, extended forward 
to the anterior margin; sulcus broad and shallow, defined in the front half; 
posterolateral slopes concave and moderately steep. Dental plates divergent; 
delthyrial cavity narrow. 
Brachial valve with fairly deep and narrow sulcus extending from the umbo 
to the anterior margin; lateral profile fairly strongly convex; greatest convexity 
at about the middle; fold low and ill-defined but indented by the sulcus; anterior 
profile broadly convex. 
Measurements in mm.— 
Length Width Thickness 
Holotype (pedicle valve) ....... 220% js-¢5 in 10.5 1.9? 
Paratype (brachial valve 116844b).......... 10.0 13.8 2.8 
“ at ps SY ETOOAAG) tects sheietee 7.9 10.8 2.3 
