PLATE XXVI. 
ce. Trapezoidal area, including central, middle j. Anterior lateral muscle scar. 
lateral, and outside lateral muscle sears. m. Pedicle muscle. 
f. Flexure line of area. p. Pedicle groove. 
g. Umbonal muscle scar. s. Median septum. 
h. Central muscle scar. vy. Visceral cavity. 
i. Transmedian muscle scar. vs. Main vascular sinus. 
LINGULELLA PHAON (Walcott) (p. 525). 
Figure 1. Ventral valve from Locality 84a, Upper Cambrian sandstone at River Junction, Houston County, Minne- 
sota, the exterior of which has been slightly injured by abrasion. U.S. Nat. Mus. Cat. No. 27406a. 
la. Dorsal valve associated with the specimen represented by figure 1. U.S. Nat. Mus. Cat. No. 27406b. 
lb. Strongly marked cast of the interior of a ventral valve from Locality 98x, Upper Cambrian sandstone near 
Eau Claire, Wisconsin. This specimen shows the broken edge of the undercut that extended back 
under the area and the deeply impressed character of the channels of the main vascular sinuses; also 
the large size of the pits or punctz on the interior of the shell. U.S. Nat. Mus. Cat. No. 27331a. 
lc. Cast of the interior of a ventral valve, the type specimen, associated with the specimen represented by fig- 
ure 1b, showing more definitely the interior features of the shell. The divided umbonal muscle scars 
and the pedicle muscle scar are drawn from an associated ventral valve. The central, middle lateral, 
and outside lateral muscle scars are not differentiated in the deeply impressed trapezoidal area (c); 
the anterior lateral muscle scars (j) are well shown. U.S. Nat. Mus. Cat. No. 27331b. 
1d. Interior cast of a dorsal valve associated with the specimen represented by figure 1b, having the same 
strongly marked characteristics as the casts of the ventral valve. The cast of the central ridge is very 
pronounced, and it is difficult to say just where the central muscle scars (h) were located. They 
appear to be, however, as indicated on the drawing, which brings them very close to the small 
anterior lateral scars. U.S. Nat. Mus. Cat. No. 27331c. 
le. Interior cast of a very strongly marked dorsal valve associated with the specimen represented by figure 
1b, in which the detailed characters are lost. The general effect of this cast is much like that of 
the interior cast of the dorsal valve of eee grandis (Pl. XX XVIII, fig. 2d). U.S. Nat. Mus. 
Cat. No. 27331d. 
OxsoLus sINoE (Walcott) (p. 415). 
Ficur® 2. Interior cast of a ventral valve. U.S. Nat. Mus. Cat. No. 2734la. 
2a. Interior of a ventral valve preserving the area and showing the undercut beneath the area. The impres- 
sions of the visceral cavity are indistinct. U.S. Nat. Mus. Cat. No. 27341b. Another specimen 
(Cat. No. 27341c) was used to restore parts of the drawing. 
2b. Cast of the interior of a ventral valve, showing a portion of the shell and the posterior portion of the main 
vascular sinuses. U.S. Nat. Mus. Cat. No. 27341d. 
2c. Dorsal valve from which the exterior layer has been remov el so as to show the edges of the oblique interior 
lamelle. U.S. Nat. Mus. Cat. No. 2734le. 
2d. Cast showing part of the interior of a ventral valve. U.S. Nat. Mus. Cat. No. 27341f. 
The specimens represented by figures 2, 2a—d are from Locality 68y, Upper Cambrian reddish brown sandstone on 
Packsaddle Mountain, Llano County, Texas. 
Figure 2e. Cast of an interior of a ventral valve. The strongly marked channel of the parietal scar and the elevated 
ridge in front of it is very characteristic of Obolus apollinis (Pl. VII). The posterior portion toward 
the area is restored from a second specimen. U.S. Nat. Mus. Cat. No. 51627a. 
2f. Cast showing part of the interior of a dorsal valve, the type specimen. The anterior lateral scars are well 
shown on another specimen. U.S. Nat. Mus. Cat. No. 51627b. 
2g. An elongate dorsal valve preserving the outer surface of the shell. U.S. Nat. Mus. Cat. No. 51627c. 
2h. Enlargement of the anterior portion of a shell to illustrate the oblique lamellz forming the strong shell. 
U.S. Nat. Mus. Cat. No. 51627d. : 
The specimens represented by figures 2e-h are from Locality 70, Upper Cambrian limestone near Morgans Creek, 
Burnet County, Texas. 
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