PLATE CII. 343 
TIuENELLA BILLINGSI (Walcott) (p. 806). 
Ficure 5. Casts of two ventral valves distorted by movement of the matrix in which they were embedded. U.S. Nat. 
Mus. Cat. Nos. 52475a and 52475b, respectively. 
5a. Exterior of a ventral valve, the type specimen, showing the character of the plications and median sinus. 
U.S. Nat. Mus. Cat. No. 52475c. 
5b. Exterior of dorsal valve, in the same layer of limestone with specimen represented by figure 5a. U.S. 
-Nat. Mus. Cat. No. 52475d. 
5c. Exterior of dorsal valve. U.S. Nat. Mus. Cat. No. 52475e. 
The specimens represented are all from Locality 28a, Middle ? Cambrian, “St. Albans formation’? 1 mile (1.6 km.) 
east of Parker’s quarry, near Georgia, Franklin County, Vermont. 
SYNTROPHIA LATERALIS (Whitfield) (p. 802). 
(Text fig. 11, p. 299.] 
Ficure 6. Partly exfoliated, transverse ventral valve and side outline. U.S. Nat. Mus. Cat. No. 33447a. 
6a. Partly exfoliated, subquadrate ventral valve and side outline. U.S. Nat. Mus. Cat. No. 33447h. 
6b. Exterior of a transverse dorsal valve. U.S. Nat. Mus. Cat. No. 33447c. 
6c. Small young dorsal valve with only a trace of the median fold. U.S. Nat. Mus. Cat. No. 33447d. 
6d. Cast of interior of dorsal valve, showing sides of spondylium, adductor scars, and vascular sinuses. U. 
Nat. Mus. Cat. No. 33447e. 
6e. Posterior view of the two valves. U.S. Nat. Mus. Cat. No. 33447f. 
6f. Portion of the interior of the ventral valve, showing the complete spondylium and the division of its sur- 
face into median (adductor) and lateral (diductor) muscular areas. > 3. Copied from Hall and 
Clarke [1893b, Pl. LXII, fig. 9]. 
6g. Oblique view of the specimen represented in figure 6f, showing the elevation of the spondylium and the 
length of its supporting median septum. Copied from Hall and Clarke [1893b, Pl. LXII, fig. 10]. 
Ss. 
The specimens represented are all from Locality 227, Ordovician limestone near Fort Cassin, Addison County, 
Vermont. 
