330 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 127 
Types.—Holotype: 116844e; figured paratypes: 116844a-d. 
Horizon and locality —Boulders in the Mystic conglomerate in Quebec, Can- 
ada: Range 6, Lot 20, 25 miles north of Mystic, Stanbridge Township. 
Discussion.—This species is fascicostellate like P. fascicostellata but differs 
in being smaller and much more convex in the profile of both valves. 
PLEURORTHIS CORINNA (Billings) 
Orthis corinna Bitirncs, Geol. Surv. Canada, Paleozoic fossils, vol. 1, p. 302, fig. 292, 1865.— 
HAtt and CrarkeE, Pal. New York, vol. 8, pt. 1, p. 217, 1892. 
Several specimens referred to this species resemble P. imbecilis (Billings) but 
differ in being proportionately wider and more strongly costellate. 
Type.—Hypotype: 66307. 
Horizon and locality—Boulders in the Mystic conglomerate in Quebec, Can- 
ada: Range 6, Lot 20, 25 miles north of Mystic, Stanbridge Township. 
PLEURORTHIS COSTELLATA Cooper, new species 
Plate 31, C, figures 11-14 
Fairly large for the genus, subrectangular in outline ; sides moderately rounded ; 
anterior margin broadly rounded ; surface marked by fine rounded costellae about 
4 to the millimeter at the front margin of an adult shell. 
Pedicle valve with moderately convex lateral profile and the greatest depth at 
about the middle; anterior profile somewhat narrowly convex in the median area 
but with long and gentle slopes to the margins; beak slightly protruding ; umbo 
narrowly swollen; median portion of valve swollen; sulcus defined only as a 
flattening of the anteromedian portion of the valve. 
Brachial valve strongly convex in lateral profile and with the greatest con- 
vexity in the median region; anterior profile strongly convex with the median 
region swollen and the lateral slopes short and steep; umbo sulcate ; sulcus short 
and narrow, dying out in the general swelling of the median region; postero- 
lateral slopes short and concave. Fold poorly defined. 
Measurements in mm.— 
Length Width Thickness 
Holotype (pedicle valve)...........sss.000 11.6 16.2 ? an 
Paratype (brachial valve 116845c).......... 11.7 15.8 2.5 
Types.—Holotype: 116845b ; figured paratype: 116845c; unfigured paratypes: 
116845a,d,e. 
Horizon and locality—Boulders in the 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 very fine and fairly even 
costellae and the lack of clearly defined fold and sulcus. It differs from P. imbe- 
cilis which is finely costellate by its more robust form. 
