PLATE LIII. 
ec. Trapezoidal area, including central, j. Anterior lateral muscle scar. 
middle lateral, and outside lateral k. Middle lateral muscle scar. 
muscle scars. 1. Outside lateral muscle scar. 
el. Cardinal scars. p. Pedicle furrow. 
h. Central muscle scar. y. Visceral area. 
i. Transmedian muscle scar. vs. Vascular sinus. 
DicELLomus NANus (Meek and Hayden) (p. 573). 
Ficures 1 and la. Exterior and side views of ventral valves varying in outline. U.S. Nat. Mus. Cat. Nos. 51903m 
and 51903a, respectively. 
1b. Interior of a ventral valve. U.S. Nat. Mus. Cat. No. 51903b. 
lc. Natural cast of an interior of a ventral valve. U.S. Nat. Mus. Cat. No. 51903c. 
1d. Interior of a ventral valve. U.S. Nat. Mus. Cat. No. 51903d. 
le. Natural cast of an interior ofa ventral valve. U.S. Nat. Mus, Cat. No. 51903e. 
lf. Partly exfoliated ventral valve. U.S. Nat. Mus. Cat. No. 519038. 
lg and lh. Exterior and side views of dorsal valves varying in outline. U.S. Nat. Mus. Cat. Nos. 51903g 
and 51903h, respectively. 
li. Interior of a dorsal valve. U.S. Nat. Mus. Cat. No. 519031. 
1j. Natural cast of interior of a dorsal valve. U.S. Nat. Mus. Cat. No. 51903). 
1k. Natural cast of interior of a dorsal valve with large cardinal scars. U.S. Nat. Mus. Cat. No. 51903k. 
11. Interior of a dorsal valve showing area. U.S. Nat. Mus. Cat. No. 519031. 
The specimens represented by figures 1, lal are from Locality 170, Middle Cambrian limestone about 10 miles 
south-southeast of Bald Mountain, Bighorn Mountains, Wyoming. 
Fieure 2. Interior of a ventral valve from Locality 355d, Upper Cambrian sandstone, Black Hills, South Dakota. 
U.S. Nat. Mus. Cat. No. 1025a. Type specimen, figured by Hayden [1862, fig. 2b, p. 73] as Obolella 
nana, and by Walcott [1899, Pl. LX, fig. 3d] as Dicellomus nanus. 
2a. Partly exfoliated dorsal valve associated with the shell represented by figure 2. U.S. Nat. Mus. Cat. 
No. 1025b. Specimen figured by Hayden [1862, fig. 2a, p. 73] as Obolella nana, and by Walcott 
[1899, Pl. LX, fig. 3b] as Dicellomus nanus. 
3. Exfoliated ventral valve from Locality 152a, Upper Cambrian sandstone on Dry Creek, Gallatin County, 
Montana. U.S. Nat. Mus. Cat. No. 51908a. 
3a. Natural cast of an interior of a ventral valve from Locality 170, Middle Cambrian limestone 10 miles south- 
southeast of Bald Mountain, Bighorn Mountains, Wyoming. U.S. Nat. Mus. Cat. No. 51903n. 
3b. Dorsal valve associated with the shell represented by figure 3a. U.S. Nat. Mus. Cat. No. 519030. 
3c and 3d. Exteriors of dorsal valves associated with the shells represented by figure 3. U.S. Nat. Mus. Cat. 
Nos. 51908b and 51908c, respectively. 
DiIcELLOMUS APPALACHIA Walcott (p. 572). 
Fieure 4. Exterior of a ventral valve from Locality 119, Middle Cambrian limestone collected somewhere in either 
Jefferson or Hawkins County, Tennessee. U.S. Nat. Mus. Cat. No. 51926a. 
4a. Cast of interior of a ventral valve, the type specimen, from Locality 89, Middle Cambrian limestone in 
Murphrees Valley, Blount County, Alabama. U.S. Nat. Mus. Cat. No. 51928a. 
4b. Exterior of a ventral valve from Locality 90a, Middle Cambrian shales at Cedar Bluff, Cherokee County, 
Alabama. U.S. Nat. Mus. Cat. No. 51939a. 
4c. Dorsal valve from Locality 16, Middle Cambrian limestone in Blountsville Valley, Blount County, Ala- 
bama. U.S. Nat. Mus. Cat. No. 51937a. 
4d. Partly exfoliated dorsal valve associated with the shell represented by figure 4. U.S. Nat. Mus. Cat. 
No. 51926b. 
4e. Partly exfoliated dorsal valve, associated with the shell represented by figure 4a, showing visceral area, 
muscle scars, etc. U.S. Nat. Mus. Cat. No. 51928b. 
4f. Partly exfoliated dorsal valve, from Locality 91, Middle Cambrian shales at Cedar Bluff, Cherokee 
County, Alabama. U.S. Nat. Mus. Cat. No. 51936a. 
4g. Partly exfoliated dorsal valve from Locality 103a, Middle Cambrian shale on the road from Rogersville 
to Dodson Ford, Hawkins County, Tennessee. U.S. Nat. Mus. Cat. No. 51938a. 
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