PART I CHAZYAN AND RELATED BRACHIOPODS—COOPER 331 
PLEURORTHIS FASCICOSTELLATA Cooper, new species 
Plate 31, E, figures 22-27 
Shell large, transversely elliptical in outline; valves subequally convex ; hinge 
slightly narrower than the greatest shell width which is at about the middle; 
lateral margins gently rounded; anterior margin broadly rounded. Surface 
marked by costellae of unequal size, about 8 in 5 mm. at the front margin. 
Costellae tending to bundle in groups of 3, thus giving a fascicostellate appearance. 
Pedicle valve moderately convex in lateral profile with the greatest convexity 
just posterior to the middle; anterior profile broadly convex; umbo narrowly 
convex and produced anteriorly as a faint swelling; sulcus broad, ill defined ; 
lateral and posterolateral slopes gently concave. Interior of pedicle valve with 
short dental plates and narrow delthyrial cavity. 
Brachial valve with moderately convex lateral profile, most convex in the pos- 
terior third; anterior profile broadly convex; sulcus narrow and deep at the 
umbo and continuing anteriorly for two-thirds the length where it merges into 
the ill-defined fold. Lateral slopes gently concave. Brachial valve as defined for 
the genus. 
Measurements in mm.— 
Length Width Thickness 
Holotype (pedicle valve) .................. 18.0 28.1 2a 
Paratype ( “ SH TTGSAT DOES lhe! 12.7 19.7 2a 
a (brachial valve 11684Ic).......... 16.8 27.8 3.5 
rs NY Pen pELOSATC,) ager Paphos 14.2 23.3 3.0 
Types.—Holotype: 117982; figured paratypes: 116841b-d,g,i; unfigured para- 
types: 116841a,e,f,h. 
Horizon and locality—Boulders in Mystic conglomerate in Quebec, Can- 
ada: In Stanbridge Township, Range 6, Lot 20, 24 miles north of Mystic; 
Range 6, Lot 21, 2.6 miles north and 4 mile east of Mystic. 
Discussion.—This species is characterized by its strongly fasciculate ornamen- 
tation and its large size. Indications are that the species attains a width of nearly 
2 inches. The large shells are fairly strongly sulcate anteriorly and were origi- 
nally mistaken for Strophomena although the ornamentation is not in accord 
with such an identification. Pleurorthis fascicostellata is more transverse than 
P, imbecilis and attains a much larger size. 
PLEURORTHIS IMBECILIS (Billings) 
Plate 32, B, figures 2-4 
Strophomena imbecilis Bittincs, Geol. Surv. Canada, Paleozoic fossils, vol. 1, p. 219 (adv. 
sheets, 1862), 1865. 
Shell elliptical in outline ; surface marked by fine, narrowly rounded costellae, 
3 to 4 in I mm. at the front margin. 
Pedicle valve with lateral profile strongly convex in umbonal region but flat 
anteriorly ; anterior profile broadly convex; sulcus broad and deep, originating 
about 3 mm. anterior to the beak; sulcus bounded by 2 fairly strong plications ; 
