PLATE 
. Central lateral space of area. 
- Outer lateral space of area. 
XVII. 
m. Pedicle muscle sear. 
p. Pedicle groove. 
, © Trapezoidal area, including central, ps. Parietal band. 
middle lateral, and outside lateral s. Median septum. 
muscle scars. v. Visceral cayity. 
f. Flexure line of area. vs. Main vascular sinus. 
g. Umbonal muscle scar. ivs. Lateral branches of vascular system. 
h. Central muscle scar. pvs. Peripheral branches of the vascular 
. Transmedian muscle scar. 
. Anterior lateral muscle scar. 
. Middle lateral muscle scar. 
system. 
x. Heart-shaped cavity. 
z. Same as ivs of the ventral valve. 
Ped PUL esol bas 
. Outside lateral and middle lateral 
muscle scars. 
LINGULELLA ACUTANGULA (Roemer) (p. 474). 
Figure 1. Ventral valve in limestone, from which the shell is partly exfoliated. Locality 69, Upper Cambrian, 
Honey Creek, Burnet County, Texas. U.S. Nat. Mus. Cat. No. 27412a. The area of this shell is 
enlarged in figure 1d. 
la. Cast of the interior of a ventral valve from Locality 353¢, Upper Cambrian sandstone, Llano County, Texas, 
showing concentric lines of tubercles that fill the puncte of the interior surface of the shell. U.S. 
Nat. Mus. Cat. No. 35240a. The area of this shell is enlarged in figure 1b. 
1b. Enlargement of cast of area and pedicle groove of figure la. U.S. Nat. Mus. Cat. No. 35240a. 
le. Cast of interior of ventral valve, from same locality as the specimen represented in figures la and 1b, in 
which the cast of the visceral cavity and muscle scars is unusually prominent. U.S. Nat. Mus. Cat. 
No. 35240b. Copied from Walcott [1898b, Pl. XX VII, fig. 6]. 
1d. Enlargement of the cast of the area of the specimen represented by figure 1. 
274124. 
le. Ventral valve in sandstone from Locality 68, Upper Cambrian, Packsaddle Mountain, Llano County, 
Texas. U.S. Nat. Mus. Cat. No. 27410a. : 
1f. Cast of the interior of a ventral valve showing the muscle scars unusually well preserved. 
Upper Cambrian sandstone, Llano County, Texas. U.S. Nat. Mus. Cat. No. 35240c. 
Walcott [1898b, Pl. XXVIII, fig. 1]. 
lg. Cast of the interior of a ventral valve from Locality 69, Upper Cambrian limestone, Honey Creek, Burnet 
County, Texas. U.S. Nat. Mus. Cat. No. 27412b. 
lh. Cast of the interior of a dorsal valve from Locality 3&3¢e, Upper Cambrian sandstone, Llano County, Texas, 
in which the interior lateral branches (z) of the vascular system are shown. U.S. Nat. Mus. Cat. 
No. 35240d. Copied from Walcott [1898b, Pl. XX VIII, fig. 2]. 
li. Cast of the interior of a dorsal valve from Locality 69, Upper Cambrian limestone, Honey Creek, Burnet 
County, Texas. U.S. Nat. Mus. Cat. No. 27412c. 
lj. Cast of the interior of a dorsal valve which shows that the punctate surface of the interior of the shell did 
not extend to the outer border. From Locality 353e, Upper Cambrian sandstone, Llano County, 
Texas. U.S. Nat. Mus. Cat. No. 35240e. 
1k. Cast of the interior of a dorsal valve from Locality 68e, Upper Cambrian sandy limestone, Packsaddle 
Mountain, Llano County, Texas. U.S. Nat. Mus. Cat. No. 27410b. 
1]. Enlargement of a portion of a ventral valve showing the outer surface in front of the line a/” a” and the 
surface of one of the inner layers of the shell back of a’ a’’. U.S. Nat. Mus. The specimen from 
which this figure was drawn can not be located; even the locality is unknown. 
Im. Median, longitudinal, thin section through the central portion of a ventral valve; os=thin outer layer 
without traces of structure; is=obliquely laminated inner layer; il=interior lamella produced 
by the thickening of the shell. U.S.-Nat. Mus. Cat. No. 27410c. 
In. Central portion of a transverse median thin section; os=thin outer layer; is=obliquely laminated inner 
layer. U.S. Nat. Mus. Cat. No. 27410d. 
lo. Median longitudinal section through the posterior portion of a ventral valve. Lettering same as for figure 
Im. U.S. Nat. Mus. Cat. No. 27410e. 
U.S. Nat. Mus. Cat. No. 
Locality 358¢e, 
Copied from 
The specimens thin-sectioned for reproduction in figures 1m, In, and lo came from Locality 68e, Upper Cambrian 
sandy limestone, Packsaddle Mountain, Llano County, Texas. 
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