492 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 127 
Brachial valve slightly convex in lateral profile; gently and broadly convex 
in anterior profile; tongue short and narrowly rounded; sulcus deep, originating 
at the umbo; flanks bounding sulcus gently convex. 
Measurements in mm.—Hypotype (24207a), length 1.4, brachial length 1,5, 
midwidth 2.9, hinge width 3.3, thickness 1.2. 
Types.—Figured hypotypes: 24207a, 4891 1a,b. 
Horizon and locality.—Platteville formation in Minnesota: At Minneapolis. 
Same formation in Illinois: At Dixon, Dixon (15’) Quadrangle. 
Discussion.—This species is distinguished by its considerable width even in 
adult form, its fairly strong, sharp, and regular costae, and low fold. It has been 
the practice to place this species in the synonymy of S. halli, but it is actually a 
much smaller shell, with less numerous costae on the flanks, much lower fold, 
and a narrower and shallower sulcus. The Minnesota species is also suggestive 
of S. perfectus from the Bromide formation (Pooleville member), but that is a 
narrower species with more distant costae and stronger fold and sulcus. 
SKENIDIOIDES BILLINGSI Schuchert and Cooper 
Scenidioides billingsi SCHUCHERT and Cooper, Mem. Peabody Mus. Nat. Hist., vol. 4, pt. 1, 
p. 72, pl. 10, figs. 6, 8, 10-14, 1932. 
Types.—Holotype: Y.P.M. 20138. 
Horizon and locality—Rockland formation, Paquette Rapids in Canada: On 
Ottawa River, Ontario. 
SKENIDIOIDES CONVEXUS Cooper, new species 
Plate 99, E, figures 24-34 
Shell of about medium size for the genus, width about twice the length ; cardi- 
nal extremities acutely angular; sides gently rounded and directed obliquely 
toward the middle of the valve; anterior margin truncated ; anterior commissure 
deeply sulcate. Surface costate, costae numbering 6 to 9 on the flanks and gen- 
erally 5 on the fold. 
Pedicle valve moderately to fairly strongly convex in lateral profile; anterior 
profile moderately and fairly evenly convex, the median region only moderately 
carinated by the fold. Fold formed by a fascicle of 5 costae, 2 bifurcating at the 
beak, 2 bifurcating just anterior to the beak, and the fifth implanted or bifurcated 
between the latter 2. One of the median ribs usually swollen and forming the 
crest of the fold. Flanks slightly swollen and merging gently into the fold; inter- 
area long and apsacline. 
Brachial valve flat in lateral profile in the posterior half but bent toward the 
pedicle valve in the anterior half; anterior profile almost flat to gently convex ; 
sulcus narrow and deep, occupied by 2 costae on its sides and 2 or 3 implanted 
at the front. Flanks narrowly but gently swollen; posterolateral areas flat to 
gently concave. 
Interior of pedicle valve with short and shallow free spondylium; brachial 
