40 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 134 
3 mm. anterior to the beak, deepening gradually but with nearly 
parallel sides to the place of geniculation; sulcus on trail somewhat 
shallower and wider but very pronounced nevertheless. Flanks bound- 
ing sulcus on visceral area moderately swollen, most so at the place 
of geniculation; slopes of flanks long and gentle; region about car- 
dinal extremities flattened. Trail strongly bilobed in anterior profile, 
with posterolateral portions nearly flat in profile. Beak small, strongly 
incurved. 
Brachial valve fitting closely into the pedicle valve to make a nar- 
row or thin visceral cavity; umbonal region sulcate; fold originating 
just anterior to the umbonal sulcus, low, somewhat narrowly rounded 
to subangular, strongest just before geniculation, less well defined 
anterior to geniculation. Flanks bounding fold gently concave. Two 
spines just anterior to geniculation moderately long, strongly di- 
vergent. 
Measurements in mm.—Holotype, length 21.1, surface length 34+, 
hinge width based on halfrmeasure 35.8+, midwidth (based on half 
measure) 30.8, thickness 4.9, height 13.2; paratype, U.S.N.M. 125337, 
length 19.2, surface length 36.5, hinge width 33+, midwidth 30.3; 
paratype, U.S.N.M. 125343, length 18, surface length 33+, hinge 
width 35+, midwidth 209.5. 
Types.—Holotype, U.S.N.M. 125339; figured paratypes, U.S. 
N.M. 125337, 125338, 125340, 125342, 125343, 125345; unfigured 
paratypes, U.S.N.M. 125341, 125344. 
Localities —638; F; L8; Pio; S4o, S43, S46, S50, S103; U.O. 
2201-2, 2202-2, 2204-3. 
Discussion—This species is characterized by its transverse out- 
line and strong geniculation. It is smaller and more strongly 
geniculated than M. multistriata (Meek) of the Word and Phos- 
phoria formations. It is larger and more strongly geniculated 
than M. geniculata Girty of the Park City formation. The Oregon 
species is similar to specimens of a Muirwoodia that occurs in 
abundance in the bituminous limestones of the lower Word forma- 
tion in the Glass Mountains. The Texas species has a deep sulcus 
and a long trail but none of the specimens in the national collection 
appears to be as strongly geniculated or to have as long and prom- 
inent.a trail as the Oregon species. 
Of European species of Muirwoodia the Oregon species is most 
like M. mammatus (Keyserling) but the latter is a variable species. 
The specimens figured by Tschernyschew (1902, pl. 35, figs. 4 and 
5) are much smaller and with a shallower sulcus than the Oregon 
specimens. The forms figured by Chao (1927, figs. 10-14) are 
