30 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 134 
visceral disk region, a deep sulcus, and strongly rounded flanks. It 
differs from M. posteroreticulata, new species, in size, in being shal- 
lower, and in having a less strongly reticulate visceral disk. This 
species is uncommon in the Oregon section. 
PROBOLIONIA POSTERORETICULATA Cooper, new species 
Plate 7A, figures 1-23; text figure 2A 
Shell of about medium size for the genus, with length and width 
about equal; sides gently rounded and sloping gently toward the 
valve middle; anterior margin gently rounded to straight. Visceral 
region of both valves coarsely reticulate; trail marked by closely 
crowded costellae. Spines forming a horizontal row of 4 near mid- 
dle, 1 on flank near ear and I on flank near sulcus. 
Pedicle valve with visceral region occupying about one-third the 
surface length, strongly reticulated and gently convex in lateral pro- 
file; geniculation strong; trail long, moderately convex in lateral 
profile. Sulcus originating 4 or 5 mm. anterior to the beak, narrow 
and moderately deep throughout its extent to the front margin. 
Flanks bounding sulcus narrowly rounded and with steep lateral 
slopes. Beak small; umbo narrow and gently swollen. 
Brachial valve with visceral region strongly and coarsely reticu- 
lated ; valve short, moderately deep, marked medianly by an obscure, 
low fold. Inner flange long and prominent. 
Measurements in mm. 
a a 
oa p=) q = 
am 8 a ae fH) Bee 
Holoty pen. 25) owas ee 23.0 12.4 15.2216 7.6 ? 8.0 
Paratype (1253606b) ..... 24.0 13:2 TAM) 1616 8.0 Po, as 
s (Gt2536G0) Seen. 24.0 13.7 17:6) 7 TA:6. TOON ? ? 
. (ST25360))) Have stale 25.0 14.6 TOM.) TALS 0.8 ? 9.0 
a CEZ5S97 A) ake ca 22.0+ 13.2+ ? 13.5 ? 53° 70 
ar (CTe8a07 TOS ee 24.5 14.4 14.6) “134 8.7 res 
Types.—Holotype, U.S.N.M. 125369; figured paratypes, U.S.N.M. 
1253606a-f,j, 125367, 125374, 125550; unfigured paratypes, U.S.N.M. 
125366g-i, 125375, 125455. 
Localities —638; Pro; S12, S29, S30, S40, S46, S49, S69, S87, 
S103, S106; U.O. 2201-4, 2202-2, 2202-5, 2216-1. 
Discussion.—This is the most common marginiferoid in the Oregon 
Permian, being most abundant at S103. The species is characterized 
