PLATE XC. 
a. Cardinal area. d’. Cast of umbonal cavity. 
a’. Anterior adductor impressions. j. Cardinal process. 
b. Dental sockets. o. Ovarian areas. 
e. Crura. ys. Vascular sinus. 
BILLINGSELLA RETROFLEXA (Matthew) (p. 761). 
Figure 1. Cast of the exterior of a rather coarsely costate ventral valve. University of Toronto, Canada. 
la. Cast of the exterior of a finely costate ventral valve. University of Toronto, Canada. 
lb. Cast of the exterior of a small, compressed ventral valve, with a strongly marked mesial depression. 
University of Toronto, Canada. 
le. Matrix of the exterior of a large dorsal valve. University of Toronto, Canada. Specimen figured by 
Matthew [1903, Pl. X, fig. 2d]. 
1d. Cast of the exterior and side outline of a dorsal valve. University of Toronto, Canada. 
le. Enlargement of the coste and radiating striz, as shown in the matrix of the anterior portion of the shell. 
University of Toronto, Canada. 
1f. Cast of a partly exfoliated dorsal valve. University of Toronto, Canada. 
lg. Cast of the interior of a ventral valve. University of Toronto, Canada. Specimen figured by Matthew 
[1895, PI. II, fig. lb] as Clitambonites (Gonambonites) plana retroflexa. 
1h. Cast of a portion of the interior of a ventral valve, in which portions of the cast of the outer surface of the 
shell are preserved. University of Toronto, Canada. Type specimen figured by Matthew [1895, 
Pl. II, fig. la] as Clitambonites (Gonambonites) plana retroflexa. 
The specimens represented are all from Locality 307d, Middle Cambrian sandy limestone on Young (McFees) 
Point, near George River, Cape Breton, Nova Scotia. 
BILLINGSELLA ROMINGERI (Barrande) (p. 762). 
Ficures 2, 2a-d. Ventral, dorsal, side, front, and posterior views of the type specimen in which the two valves are 
united, from Locality 345f, Middle Cambrian Etage C, near Skrej, Bohemia. Figures 2, 2a-d are copied from 
Barrande [1879a, Pl. LXII, figs. II: 3a, 3c, 3b, 3d, and 3e, respectively]. 
Ficure 2e. Exterior and side views of a ventral valve. U.S. Nat. Mus. Cat. No. 52267a. 
2f. Matrix of ventral valve in which the area is preserved. U.S. Nat. Mus. Cat. No. 52267b. The specimen 
is on the same hand fragment with the ventral valve shown in figure 2h. 
2g, 2h, and 2i. Casts of the interiors and side outlines of ventral valves. U.S. Nat. Mus. Cat. Nos. 52267c, 
52267d, and 52267e, respectively. The specimen represented by figure 2h is on the same hand frag- 
ment as the one shown in figure 2f. 
2j. Cast of interior and side view of a dorsal valve. U.S. Nat. Mus. Cat. No. 52267f. 
2k. Cast of the interior and side view of a dorsal valve showing very strong muscular impressions. U.S. Nat. 
Mus. Cat. No. 52267g. : 
21. Cast of the interior of a dorsal valve. U.S. Nat. Mus. Cat. No. 52267h. 
2m. Cast of the interior of a very small ventral valve. U.S. Nat. Mus. Cat. No. 522671. 
2n, 2n’, and 2n”’. Top, posterior, and side views of the cast of the interior of a ventral valve in which the 
vascular trunks are readily traced to the back of the interior of the beak. U.S. Nat. Mus. Cat. No. 
52267}. 
The specimens represented by figures 2e—n are from Locality 345, Middle Cambrian shale on the Dlouhé Hora, 
above the brook of Shirov, near Skrej, Bohemia. 
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