PLATE LXXIII. 
el. Cardinal scar. s. Median septum. 
fa. False area. vs. Main vascular sinus. 
fp. False pedicle groove. x’. Cast of foraminal aperture. 
h. Central sear. x’’. Cast of apical callosity. 
ACROTRETA NICHOLSONI Davidson (p. 696). 
[Pl. LX XIII, figs. 6, 6a-b.] 
* Figure 1. Top and side views of a compressed ventral valve, the type specimen. Copied from Davidson [1868, Pl. 
XVI, fig. 14]. : 
la and 1b. Compressed ventral valves. Copied from Davidson [1871, Pl. XLIX, figs. 38 and 39, respectively]. 
le. Exterior of dorsal valve. Copied from Davidson [1868, Pl. XVI, fig. 15]. 
1d. Interior of dorsal valve. Copied from Davidson [1868, Pl. XVI, fig. 16]. 
The specimens represented by figures 1, la-d are from Locality 316, Lower Ordovician, Upper Llandeilo? ‘‘Black 
shales’’ at Dobbs Linn, near Moffat, in Dumfriesshire, Scotland. 
Fiaures le, lf, and lg. Front, side, and back views of ventral valve, from Locality 316a, Lower Ordovician, Llandeilo, 
at Craighead, Girvan district, Ayrshire, Scotland. 
th and li. Back and front views of a ventral valve, from Locality 316a, Lower Gd Llandeilo, at 
Balcletchie, Girvan district, Ayrshire, Scotland. 
1j. Exterior of the dorsal valve attached to the specimen represented by figures le, 1f, and lg. 
1k. Exterior of the dorsal valve attached to the specimen represented by figures 1h and 1i. 
Figures le-k are copied from Davidson [1883, Pl. XVI, figs. 21a, 21c, 21b, 22a, 22b, 21d, and 22c, respectively]. 
ACROTRETA suBcoNIcA Kutorga (p. 713). 
Fiaures 2, 2a-d. Ventral and dorsal valves. Copied from Kutorga [1848, Pl. VII, figs. 7a, 7b, 7c, 7b’, and 7c’, respec- 
tively]. The specimens represented by figures 2, 2c, and 2d are redrawn in figures 2e, 2h, and 2i, 
respectively, of this plate. 
2e. Ventral valve. Specimen figured by Kutorga [1848, Pl. VII, fig. 7a]. (See fig. 2.) 
2f and 2¢. Side and posterior views of the type specimen in Kutorga’s collection preserving both ventral 
and dorsal valves. The dorsal valve shown in these figures is the one figured by Kutorga [1848, 
Pl. VI, fig. 7c’]. A top view of it is given in figure 2i. 
2h. Posterior view of a ventral valve showing the median groove and pedicle aperture. Specimen figured 
by Kutorga [1848, Pl. VII, fig. 7b’]. (See fig. 2c.) 
2i. Dorsal valve shown in side outline by figures 2f and 2g. Specimen figured by Kutorga [1848, Pl. VII, 
fig. 7c’]. (See fig. 2d.) 
The specimens represented are from Locality 336, Ordovician, Echinospherites limestone, horizon Cla, at Popowka, 
near St. Petersburg, Russia. , 
ACROTRETA SOCIALIS von Seebach (p. 711). 
Figures 3 and 3a. Side and back views of ventral valve, the type specimen. 
3b. Interior of ventral valve. 
3c. Cast of dorsal valve. 
The specimens represented by figures 3, 3a-c are from Locality 334g, Middle Cambrian, Bornholm Island, Den- 
mark. The figures are copied from yon Seebach [1865, Pl. VIIla, figs. 14, respectively]. 
Fiaures 4 4’, 4”, and 4a, 4a’, 4a’’. Top, back, and side views of two ventral valves from Locality 8z, Middle Cambrian 
limestones of the Paradoxides elandicus zone at Borgholm, Oeland Island, Sweden. U. 8. Nat. 
Mus. Cat. Nos. 35286a and 35286b, respectively. : 
4b, 4b’, and 4b’”. Top, back, and side views of a cast of part of the interior of a ventral valve from Locality 
8w, Middle Cambrian limestones of the Paradoxides forchhammeri zone at Andrarum, 20 miles (32 km.) 
northwest of Simrishamn, Province of Christianstad, Sweden. U.S. Nat. Mus. Cat. No. 52174a. 
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