PLATE LXXXVIII. 
a’. Anterior adductor impressions. d’, Cast of umbonal cavity. 
a’’. Posterior adductor impressions. j. Cardinal process. 
e. Crura. s. Median septum. 
ep. Septal plates. 
BILLINGSELLA EXPORRECTA (Linnarsson) (p. 754). 
Fiaure 1. Exterior and side views of a ventral valve preserving the outer surface. U.S. Nat. Mus. Cat. No. 52249a. 
la. Exterior and side views of ventral valve, in which the plications are stronger than in the specimen repre- 
sented by figure 1. U.S. Nat. Mus. Cat. No. 52249b. 
1b. Dorsal valve flattened in the shale. U.S. Nat. Mus. Cat. No. 52249c. 
le. Exterior and side views of rather strongly costate dorsal valve. U.S. Nat. Mus. Cat. No. 52249d. 
1d and 1d’. Top, side, and back views of a cast of a somewhat quadrate ventral valve. U.S. Nat. Mus. Cat. 
No. 52249e. 
le. Top and side views of the cast of a transverse ventral valve. U.S. Nat. Mus. Cat. No. 52249f. Another 
specimen (Cat. No. 52249¢) was used to restore certain portions of this figure. 
lf. Interior of a dorsal valve, with strongly developed median ridge. U.S. Nat. Mus. Cat. No. 52249h. 
lg. Cast of the interior of a dorsal valve, having very strongly marked muscle scars. U.S. Nat. Mus. Cat. No. 
522491. 
th. Interior of a dorsal valve with strongly marked muscle scars. U. 8. Nat. Mus. Cat. No. 52249]. The 
specimen represented occurs on the same hand specimen with that represented by figure 1 1. 
li. Interior of a dorsal valve that should be compared with the interior represented by figure lf. U.S. Nat. 
Mus. Cat. No. 52249k. 
1j. Cast of the interior of a dorsal valve, with very strong muscle scars. U.S. Nat. Mus. Cat. No. 52249 1. 
1k. Cast of the interior of a dorsal valve, with strong muscle scars and vascular sinuses. U.S. Nat. Mus. Cat. 
No. 52249m. 
11. Cast of the interior of dorsal valve, with rather faintly marked muscle scars. U.S. Nat. Mus. Cat. No. 
52249n.° This specimen is associated on the same hand specimen with the specimen represented by 
figure 1h. 
The specimens represented are all from Locality 320b, Middle Cambrian limestones of the Agnostus levigatus zone 
at Gudhem, 12.5 miles (20.1 km.) south-southeast of Skara, Province of Skaraborg, Sweden. 
BILLINGSELLA EXPORRECTA RUGOSICOSTATA Walcott (p. 755). 
Figure 2. Top and side views of a cast of the interior of a ventral valve, the type specimen, having very strongly 
marked coste. U.S. Nat. Mus. Cat. No. 52250a. 
2a. Top and side views of a cast of the interior of a ventral valve, having strongly marked cost with fine radi- 
ating strize upon them. U.S. Nat. Mus. Cat. No. 52250b. 
2b. Top and side views of a cast of the interior of a dorsal valve. U.S. Nat. Mus. Cat. No. 52250c. 
2c. Top and side views of a cast of the interior of a dorsal valve less strongly costate than most specimens of 
this variety. U.S. Nat. Mus. Cat. No. 52250d. 
The specimens represented are all from Locality 320b, Middle Cambrian limestones of the Agnostus lxvigatus 
zone at Gudhem, 12.5 miles (20.1 km.) south-southeast of Skara, Province of Skaraborg, Sweden. 
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