PLATE CIL. 
a’, Anterior adductor scars. sp. Cast of spondylium, 
a’’. Posterior adductor scars. vs. Vascular sinus. 
s. Median septum supporting spondylium. 
SYNTROPHIA BARABUENSIS (A. Winchell) (p. 799). 
Ficure 1. Cast of the interior of a ventral valve, showing the base of the median septum that supported the spondyl- 
ium, the cast of which is clearly indicated. Locality 134a, Upper Cambrian sandstone in quarry 
1 mile (1.6 km.) southeast of the courthouse, Menomonie, Dunn County, Wisconsin. 
la. Cast of the interior of a transverse ventral valve associated with the specimen represented by figure 1, 
with posterior and side outline. U.S. Nat. Mus. Cat. No. 52473b. 
1b. Top view and side outline of a cast of a ventral valve from Locality 88, Upper Cambrian sandstone near 
Trempealeau, Trempealeau County, Wisconsin. U.S. Nat. Mus. Cat. No. 52472a. The median 
sinus is very faintly impressed; see figures lc and 1d, where it is narrower and deeper. 
le and 1d. Top views and side outlines of casts of ventral valves showing variation in form and in the breadth 
and depth of the median sinus. From Locality 810, Upper Cambrian sandstone below the Cliff 
House, Devils Lake, Sauk County, Wisconsin. U. 8. Nat. Mus. Cat. No. 52474a and 52474b, 
respectively. 
le and lg. Top views and side outlines of casts of the interior of dorsal valves, illustrating variation in form, 
median fold, and cast of spondylium. They are associated with the specimen represented by figure 
lb. U.S. Nat. Mus. Cat. Nos. 52472b and 52472c, respectively. 
1f. Top viewand side outline of acast of the interior of a dorsal valve associated with the specimen represented 
by figure le. Compare the median fold and the cast of the spondylium of this specimen with those of 
figures lc and lg. U.S. Nat. Mus. Cat. No. 52474c. 
SYNTROPHIA PRIMORDIALIS (Whitfield) (p. 803). 
Ficure 2. Cast of the interior of a ventral valve, showing cast of the spondylium, the supporting median septum and 
large vascular sinuses. U.S. Nat. Mus. Cat. No. 52486a. 
2a and 2b. Casts of the interior of ventral valves, illustrating variation in form. U. 8. Nat. Mus. Cat. Nos. 
52486b and 52486c, respectively. ; 
2c and 2c’. Top and side views of cast of the interior ofa dorsal valve. U.S. Nat. Mus. Cat. No. 52486d. 
The specimens represented by figures 2, 2a-c are from Locality 99, Upper Cambrian sandstone at Minneiska, 
Minnesota. 
Fieures 2d and 2e. Side and top views of the type specimen of the species, a dorsal valve from Locality 328k, Upper 
Cambrian, ‘‘St. Croix sandstone’’ at Roche 4 Cris Bluff, Adams County, Wisconsin. Copied from 
Whitfield [1882, Pl. X, figs. 1 and 2]. 
SYNTROPHIA PRIMORDIALIS ARGIA Walcott (p. 804). 
Figure 3. Cast of the interior of a ventral valve, the type specimen, from Locality 97, Upper Cambrian sandstone at 
Reads Landing, foot of Lake Pepin, Wabasha County, Minnesota. U.S. Nat. Mus. Cat. No. 52492a. 
SYNTROPHIA NUNDENA Walcott (p. 802). 
Ficurss 4 and 4’. Exterior and side views of a ventral valve from Locality 204, Lower Ordovician Pogonip limestone 
on eastern slope of ridge east of Hamburg Ridge, Eureka district, Nevada. U.S. Nat. Mus. Cat. No. 
1728la. Type specimen figured by Walcott [1884b, Pl. XI, fig. 7] as Triplesia calcifera. 
4a. Exterior of a dorsal valve from Locality 203, Lower Ordovician Pogonip limestone on ridge extending 
southwest from Wood Cone, Eureka district, Nevada. U.S. Nat. Mus. Cat. No. 172792. Specimen 
figured by Walcott [1884b, Pl. XI, fig. 8] as Triplesia calcifera. 
4b. Partly exfoliated ventral valve, in which the beak is either broken or partly worn away. Associated 
with the specimen represented by figure 4a. U.S. Nat. Mus. Cat. No. 17279b. 
4c. Cast of the interior ofa ventral valve, having an unusually deep and strongly marked median sinus. Local- 
ity 201a, Lower Ordovician Pogonip limestone on east slope of ridge east of Hamburg Ridge, Eureka 
district, Nevada. U.S. Nat. Mus. Cat. No. 17280a. 
4d. Partly exfoliated dorsal valve, showing the cast of the spondylium and muscle scars. U.S. Nat. Mus. 
Cat. No. 1749la. This specimen is doubtfully referred to this species. It is from Locality 329a, 
Upper Cambrian limestone on Ute Peak, in the Wasatch Mountains, east of Cache Valley, Cache 
County, Utah. 
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