PLATE CIV. 
el. Area of attachment of posterior muscles. , uc. Umbonal cavity. 
h. Central scars. vs. Main vascular sinuses. 
sp. Cast of spondylium. 
SYNTROPHIA CALCIFERA (Billings) (p. 800). 
Ficure 1. Ventral valve,probably the one represented by figure 1c. Copied from Billings [1861a, fig. 3a, p. 319]. 
la. Dorsal valve, possibly the one represented by figure le. Copied from Billings [1861a, fig. 3b, p. 319]. 
1b. Cast of a ventral valve showing spondylium. Copied from Billings [1861a, fig. 3c, p. 319]. 
le and lc’. Top and side views of ventral valve, from Locality 392j, Lower Ordovician limestone at Point 
Levis, Province of Quebec, Canada. Geol.Survey Canada. In all probability this is the specimen 
used by Billings in drawing his figure 3a [1861la, p. 319]. It may be taken as the type. 
1d. Ventral valve with gently rounded shallow sinus, with a large projection to fitin the fold of the dorsal valve. 
It is associated with the specimen represented by figure 1c. Geol. Survey Canada. 
le and le’. Top and side views of a dorsal valve from Locality 392i, Lower Ordovician limestones at 
Phillipsburg, Province of Quebec. Geol. Survey Canada. The specimen represented by figure le is 
possibly the one used by Billings in drawing his figure 3b [186la, p. 319]. 
lf. Ventral valve with side outline. U.S. Nat. Mus. Cat. No. 52476a. 
lg and lh. Top and side view of a ventral valve. U.S. Nat. Mus. Cat. No. 52476b. 
li. A smooth dorsal valve, doubtfully referred to this species. U.S. Nat. Mus. Cat. No. 52476c. 
The specimens represented by figures 1f-i are from Locality 392j, Lower Ordovician limestones at Point Levis, 
Province of Quebec, Canada. “ 
CLARKELLA MONTANENSIS (Walcott) (p. 810). 
Fieures 2 and 2’. Top and back view of a broken ventral valve. U.S. Nat. Mus. Cat. No. 11587a. 
2a and 2a’. Top and side view of a rhomboidal dorsal valve, the type specimen. U.S. Nat. Mus. Cat. No. 
11587b. 
2b. Transverse dorsal valve. U.S. Nat. Mus. Cat. No. 11587c. 
2c and 2d. Sections across the umbo of two ventral valves, showing sections of the septa supporting the 
spondylium. U.S. Nat. Mus. Cat. Nos. 11587d and 11587e, respectively. 
The specimens represented are all from Locality 302}, Ordovician limestone east of West Gallatin (Gallatin) 
River, above Gallatin, Gallatin County, Montana. 
HUENELLA ORIENTALIS (Walcott) (p. 808). 
Ficure 3. Top and side views of a ventral valve, the type specimen. U.S. Nat. Mus. Cat. No. 52564a. 
3a. Top and side views of an exfoliated dorsal valve. U.S. Nat. Mus. Cat. No. 52564b. 
3b. Partly exfoliated dorsal valve. U.S. Nat. Mus. Cat. No. 52564c. ~ 
The specimens represented are all from Locality C64, Upper Cambrian limestone 2.7 miles (4.4 km.) southwest of 
Yenchuang, Sintai district, Shantung, China. 
SYNTROPHIA ORTHIA Walcott (p. 803). 
Ficure 4. Ventral valve, the type specimen, from Locality C56, Upper Cambrian Chaumitien limestone at Pagoda 
Hill, 1 mile (1.6 km.) west of Tsinan, Shantung, China. U.S. Nat. Mus. Cat. No. 52562a, 
4a and 4a’. Exterior of ventral valve, with side outline, from Locality €64, Upper Cambrian limestone 2.7 miles 
(4.4 km.) southwest of Yenchuang, Sintai district, Shantung, China. U. 8S. Nat. Mus. Cat. No. 
52563a. 
4b ana 4b’. Top and side views of a dorsal valve from Locality €54, Upper Cambrian Chaumitien limestone 
0.66 mile (1.1 km.) west of Tsinan, Shantung, China. U.S. Nat. Mus. Cat. No. 52561a. 
SWANTONIA ANTIQUATA (Billings) (p. 797). 
Fictres 5, 5a, and 5b. Top, side, and back views of the cast of a ventral valve from Locality 319e, Lower Cambrian 
sandy shales about 2 miles (3.2 km.) east of Swanton, Franklin County, Vermont. U.S. Nat. Mus. 
Cat. No. 15363a. This specimen was figured by Walcott [1886b, Pl. VII, fig. 8] as Camarella antiquata. 
