PLATE XXI. 
f. Flexure line of area. p. Pedicle groove. 
g. Umbona] muscle sear. s. Median septum. 
h. Central muscle scar. v. Visceral cavity. 
j. Anterior lateral muscle sear. : vs. Main vascular sinus. 
LINGULELLA PERATTENUATA (Whitfield) (p. 523). 
Figure 1. Cast of the interior of a ventral valve from Locality 355, Middle Cambrian sandstones on Red Canyon Creek, 
Black Hills, South Dakota. U.S. Nat. Mus. Cat. No. 27423a. 
la. Cast of the interior of a ventral valve from Locality 74, Middle Cambrian sandstone, head of Nunkoweap 
Valley, Grand Canyon of the Colorado, Arizona. U.S. Nat. Mus. Cat. No. 51796a. 
1b. Narrow, partly exfoliated ventral valve, with the apex rounded, from Locality 355, Middle Cambrian 
sandstones on Red Canyon Creek, Black Hills, South Dakota. U.S. Nat. Mus. Cat. No. 24589a. 
Type specimen, figured by Whitfield [1880, Pl. II, fig. 9] as Lingulepis perattenuatus. 
Je. Cast of the interior of a ventral valve associated with the valve represented by figure 1. U.S. Nat. Mus. 
Cat. No. 27423b. This specimen occurs on the same hand specimen with figured specimens of Lin- 
gulella (Lingulepis) acwminata. 
1d. Cast of the interior of a dorsal valve associated with the valves represented by figureslandlc. U.S. Nat. 
Mus. Cat. No. 27423c. 
le. Cast. of the interior of a ventral valve from Locality 67¢, Upper Cambrian sandstone, Tatur Hill, Burnet 
County, Texas. U.S. Nat. Mus. Cat. No. 27424a. 
lf. Partly exfoliated dorsal valve from Locality 68y, Upper Cambrian sandstone, Packsaddle Mountain, Llano 
County, Texas. U.S. Nat. Mus. Cat. No. 27422a. 
lg. Partly exfoliated dorsal valve associated with the valve represented by figure 1f. U.S. Nat. Mus. Cat. 
No. 27422b. 
lh. Cast of the interior of a dorsal valve associated on the same hand specimen with the valve represented by 
figure Ib. U.S. Nat. Mus. Cat. No. 24589b. Specimen figured by Whitfield [1880, Pl. II, fig. 8] as 
Lingulepis perattenuatus. 
li. Cast of the interior of a dorsal valve associated with the valves represented by figures 1b and 1h. U.S. Nat. 
Mus. Cat. No. 24589c. Specimen figured by Whitfield [1880, PI. II, fig. 7] as Lingulepis perattenuatus. 
LINGULELLA simILtis (Walcott) (p. 532). 
Ficure 2. Cast of the interior of a ventral valve, the type specimen, from Locality 88a, Middle Cambrian limestone, 
near Deadwood, Black Hills, South Dakota. U.S. Nat. Mus. Cat. No. 27339a. 
2a. Cast of the interior of a dorsal valve associated with the specimen represented by figure 2. U.S. Nat. Mus. 
Cat. No. 27339b. 
2b and 2c. Partly exfoliated ventral and dorsal valves, respectively, associated with the specimens repre- 
sented by figures 2 and 2a. U.S. Nat. Mus. Cat. Nos. 27339c¢ and 27339d, respectively. 
2d. Small, strongly convex dorsal? valve doubtfully referred to this species, from same locality and bed of 
limestone as the specimens represented by figures 2, 2a-c. U.S. Nat. Mus. Cat. No. 27339e., 
2e. Cast of the interior of an unusually convex, elongate ventral valve. U.S. Nat. Mus. Cat. No. 27340a. 
2f. Cast of the interior of a ventral valve that narrows toward the frontal margin. U.S. Nat. Mus. Cat. No. 
27340b. 
2g and 2h. Casts of two dorsal valves that show variation in outline. U.S. Nat. Mus. Cat. Nos. 27340c and 
27340d, respectively. 
2i and 2]. Casts of associated ventral and dorsal valves that approach most closely the typical forms from the 
Black Hills. U.S. Nat. Mus. Cat. Nos. 27340e and 27340f, respectively. 
The specimens from which figures 2e to 2j are drawn are all from Locality 97a, Upper Cambrian sandstone, at Win- 
field, Wisconsin. 
Fieures 3 and 3a. Casts of the interior of ventral valves from Locality 106, Middle Cambrian shaly sandstones of the 
Rome formation, in Bays Mountains, 10 miles (16.1 km.) southeast of Knoxville, Tennessee. U.S. Nat. 
Mus. Cat. Nos. 27367a and 27367b, respectively. 
3b. Cast of the interior of a ventral valve from Locality 10a, Middle Cambrian, at the same general locality as 
the specimens represented by figures 3 and 3a. U.S. Nat. Mus. Cat. No. 27326a. 
3c and 3d. Casts of the interior of dorsal valves associated with the ventral valves represented by figures 3 and 
3a. U.S. Nat. Mus. Cat. Nos. 27367c and 27367d, respectively. 
~I 
a 
