338 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 127 
Types.—Holotype: 116849a; figured paratypes: 116849b,d,f; unfigured para- 
types : 116849¢,e. 
Horizon and locality—Boulders in the Mystic conglomerate in Quebec, Can- 
ada: Stanbridge Township, Range 6, Lot 21, 2.6 miles north and 4 mile east 
of Mystic. 
Discussion.—This species is characterized by its wide hinge and brachial valve 
of low convexity. It differs from H. porcias (Walcott) in having a stronger cos- 
tellation and less convex brachial valve. The delthyrial cavity of the Quebec 
species is also much smaller than that of H. porcias. 
Family ORTHIDIELLIDAE Ulrich and Cooper, 1936 
Small Orthacea in which the simple cardinal process is cemented to the 
brachiophores by deposits of shell substance. 
Genus ORTHIDIELLA Ulrich and Cooper, 1936 
Orthidiella ULricu and Cooper, Journ. Paleont., vol. 10, No. 7, p. 621, 1936; Geol. Soc. Amer. 
Special Pap. 13, p. 108, 1938. 
ORTHIDIELLA CARINATA Ulrich and Cooper 
Orthidiella carinata ULRicH and Cooper, Geol. Soc. Amer. Special Pap. 13, p. 108, pl. 16A, 
figs. I, 5, 6, 1938. 
Type.—Holotype : 91206. 
Horizon and locality—Same as for O. extensa, below. 
ORTHIDIELLA COSTELLATA Cooper, new species 
Plate 30, C, figures 12-19 
Shell large for the genus, subquadrate in outline; hinge forming the widest 
part; lateral margins very gently convex; anterior margin strongly convex ; car- 
dinal extremities acute; surface marked by fine, rounded costellae that bifurcate 
2 or 3 times but are not fasciculate; about 3 costellae to the millimeter. 
Pedicle valve subcarinate medially but the carina becoming lost in the swollen 
anterior region; lateral profile gently convex; anterior profile forming a broad 
triangle, fairly sharp at the apex and with long gently sloping sides; umbonal 
and lateral slopes long and gently concave. Pedicle interior with strong, thick 
teeth; dental plates nearly obsolete; deep fossette located just below teeth; 
delthyrial cavity deep and wide; diductor muscle scars long and slender, extend- 
ing to points anterior to the middle; interarea apsacline; pseudodeltidium nar- 
rowly arched, imperforate. 
Brachial valve moderately convex in lateral profile; anterior profile bilobed, 
the lobes low and indented by the median sulcus; umbo sulcate, sulcus continued 
anteriorly to the front margin where it is fairly wide; areas bounding sulcus 
gently swollen; posterolateral slopes gently convex. Brachial interior with 
median ridge extending to the middle, low and rounded and separating deeply 
