PLATE LXXVII. 255 
ACROTRETA GEMMULA Matthew—Continued. 
Fiewres 5c and 5c’. Top and side views of a ventral valve associated with the specimen represented by figure 5, 
that has a more erect posterior side than most specimens. U.S. Nat. Mus. Cat. No. 52088d. 
5d and 5d’. Back and side views of a ventral valve associated with the specimen represented by figure 5, 
the surface of which has been roughened by the impress of the fine grains of sand in the matrix so 
that it resembles the surface characteristic of ‘‘ Acrotreta papillata®” Matthew=Acrothyra sera. U.S. 
Nat. Mus. Cat. No. 52088e. 
5e. Side view of a ventral valve associated with the specimen represented by figure 5, in which the posterior 
side curves backward as in Matthew’s figure of the ventral valve [1903, Pl. III, fig. 4b]. U.S. Nat. 
Mus. Cat. No. 52088f. 
5f and 5f’. Top and side views of the exterior of a dorsal valve associated with the specimen represented by 
figure 5. U.S. Nat. Mus. Cat. No. 52088¢. 
ACROTRETA BABEL Barrande (p. 676). 
Figures 6 and 6a. Top and side views of a ventral valve, the type specimen, from Locality 303, Lower Ordovician, 
Etage d3 at Kénigshof, Bohemia. Copied from Barrande [1879b, Pl. XCV, figs. vu: 2A and 2B, 
respectively]. 
6b. Cast of dorsal valve associated with specimen represented by figures 6 and 6a. Copied from Barrande 
[1879b, Pl. XCV, fig. viz: 2C]. 
ACROTRETA ? MINIMA (Barrande) (p. 695). 
Fieure 7. Top view of ventral valve, the type specimen, from Locality 303d, Lower Ordovician, Etage dl at Swarow, 
Bohemia. 
' 7a. Cast of dorsal valve associated with the specimen represented by figure 7. 
The figures are both copied from Barrande [1879b, Pl. XOV, figs. 1: 3A and 1A, respectively], who labeled 
them ‘‘Obolus ? minimus.”’ 
ACROTRETA BELTI (Davidson) (p. 678). 
Ficure 8. Cast of the interior of a compressed ventral valve, the type specimen, from Locality 305, Upper Cambrian 
Lower Tremadoc shales at Craig-y-dinas, North Wales. 
8a. Cast of the interior of a dorsal valve associated with the specimen represented by figure 8. 
The figures are both copied from Davidson [1871, Pl. L, figs. 16a and 17a, respectively]. The same specimens 
were figured by Davidson [1868, Pl. XV, figs. 27a and 26a, respectively], both as Obolella beltt. 
ACROTRETA NEBOENSIS Walcott (p. 695). 
Figure 9. Top view of ventral valve. U.S. Nat. Mus. Cat. No. 52130a. 
9a, 9a’, and 9a/’. Top, side, and back views of the exterior of a ventral valve, the type specimen. U.S. Nat. 
Mus. Cat. No. 52130b. 
9b. Exterior of a dorsal valve, showing strong median sinus. U.S. Nat. Mus. Cat. No. 52130c. 
The specimens represented are all from Locality 14t, Middle Cambrian limestone in Mount Nebo Canyon, 3 miles 
(4.8 km.) southeast of Mona, Juab County, Utah. 
AcRoTRErA Nox Walcott (p. 696). 
Fieures 10 and 10’. Top and side views of a ventral valve, the type specimen, from Locality 10v, Upper Cambrian 
shales at Fox Glen, about 8 miles (12.8 km.) east of Baraboo, Sauk County, Wisconsin. U.S. Nat. 
Mus. Cat. No. 52132a. 
