PLATE XV. 
h. Central muscle sear. j. Anterior lateral muscle scar. 
Opotus (MIcKWITZELLA) stLuRIcus (Eichwald) (p. 434). 
Figure 1. Exfoliated dorsal valve. U.S. Nat. Mus. Cat. No. 51538a. 
la. Exfoliated ventral valve. U.S. Nat. Mus. Cat. No. 51538b. 
1b. Exterior surface of shell. Copied from Mickwitz [1896, Pl. III, fig. 9]. 
Ic. Enlargement of surface of ventral valve. U.S. Nat. Mus. Cat. No. 51538c. 
The specimens represented are all from Locality 396a, passage beds between the Upper Cambrian and the Ordo- 
vician, ‘“‘glauconite sandstone,” at Leppiko, near Leetz, Esthonia, Russia. 
Oxgotus (AcriITISs) ANTIQUISSIMUS (Hichwald) (p. 437). 
(Pl. XII, fig. 3.] 
Figures 2, 2a, and 2b. Ventral, side, and dorsal views of specimen preserving the two valves nearly united. The 
outer surface of the shell is almost entirely exfoliated. U.S. Nat. Mus. Cat. No. 51534b. 
2c. Photographic enlargement of the exterior surface of the shell. U.S. Nat. Mus. Cat. No. 51534e. 
2d. Exterior of a partly exfoliated dorsal valve, with the side outline of the two valves united. Copied 
from Mickwitz [1896, Pl. III, figs. 20a—b.] 
The specimens represented are all from Locality 396d, passage beds between the Upper Cambrian and the Ordo- 
vician, ‘‘glauconite limestone,” at Domglint, in Reval, Esthonia, Russia. 
OBOLUS ELEGANS Mickwitz (p. 390). 
Fieure 3. Exterior of a ventral? valve showing a strongly marked surface. Specimen from Locality 395b, Upper 
Cambrian Obolus sandstone at Ilgast, Esthonia, Russia. U.S. Nat. Mus. Cat. No. 51535a. 
OxsoLus (BROGGERIA) SALTERI (Holl) (p. 424). 
[Pl. XIU, figs. 1, lan.] 
Figure 4. Figure of ventral valve, the type specimen, from Locality 304e, Upper Cambrian “‘Black shales” at Coal 
Hill, Malvern Hills, England. Copied from Holl [1865, p. 101, fig. 9]. The figure was also copied 
by Davidson [1866, Pl. IV, figs. 28a and 28]. 
4a and 4b. Ventral and dorsal valves occurring in Locality 310h, Upper Cambrian dark argillaceous shale 
somewhere in Skane (Provinces of Malméhus and Christianstad), Sweden. U.S. Nat. Mus. Cat. 
Nos. 1493la and 14931b, respectively. 
4c. Cast of dorsal valve from Locality 309j, Upper Cambrian argillaceous shale at Alunbruk, Oeland Island, 
Sweden. U.S. Nat. Mus. Cat. No. 51676a. 
4d. Partly exfoliated dorsal valve from Locality 309h, Upper Cambrian very dark siliceous limestone at 
Fogelsang, Province of Malméhus, Sweden. U. 8. Nat. Mus. Cat. No. 51674a. : 
Oxso.tus? pALLiaTuS Barrande (p. 407). 
Figure 5. Exterior surface of dorsal valve, the type specimen, from Locality 308c, passage beds between the Cam- 
brian and Ordovician, in the suburbs of Hof, Bavaria, Germany, one-third natural size. Copied 
from Barrande [1868a, fig. 65]. 
Oxso.tus? minor Barrande (p. 405). 
Ficure 6. Exterior surface of ventral valve, the type specimen, from Locality 308¢, passage beds between the Cam- 
brian and Ordovician, in the suburbs of Hof, Bavaria, Germany, one-third natural size. Copied 
from Barrande [1868a, fig. 68]. 
ACROTRETA INCHOANS (Barrande) (p. 690). 
Ficures 7 and 7a. Two views copied from Barrande [1868a, figs. 74 and 75] of shells from Locality 303c, passage 
beds between the Cambrian and the Ordovician, suburbs of Hof, Bavaria, Germany. Figure 7 is 
the type. 
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