PLATE VII. 
a. Central lateral space of area. k. Middle lateral muscle scar. 
a’. Outer lateral space of area. 1. Outside lateral muscle scar. 
f{. Flexure line of area. p. Pedicle groove. 
g. Umbonal muscle scar. vs. Main vascular sinus. 
h. Central muscle scar. x. Heart-shaped cavity. 
i. Transmedian muscle scar. x’. A slight depression of the bottom of the . 
j. Anterior lateral muscle scar. heart-shaped cavity. 
Oxsotus APOLLINIS Eichwald (p. 381). 
[Text figs. 4, p. 299, and 15, p. 299, and Pl. XIV, figs. 6, 6a.) 
Figures 1-6. Interiors of ventral valves illustrating variations in size, form, and position of the central, middle lateral, 
and outside lateral muscle scars. From Locality 94, Upper Cambrian Obolus sandstone, at Jaggowal, 
east of Reval, Esthonia, Russia. U.8&. Nat. Mus. Cat. Nos. 51524a-t, respectively. 
7 and 8. Interiors of ventral valves from Locality 395b, Upper Cambrian Obolus sandstone, at Ilgast, Esthonia, 
Russia, showing variation in form and arrangement of the muscle scars, central depression, ete. U.S. 
Nat. Mus. Cat. Nos. 51525a and 51525b, respectively. 
10, 11, and 12. Interiors of ventral valves from Locality 94, Upper Cambrian Obolus sandstone, at Jaggowal, 
Esthonia, Russia, showing variation in the form and arrangement of the muscle scars, central depres- 
sion, etc. U.S. Nat. Mus. Cat. Nos. 35238a, 35238b, and 35238c, respectively. Specimens figured 
by Walcott [1898b, Pl. X XVI, figs. 3-5, respectively]. 3 
13. Fragment of the interior of a ventral valve from Locality 94, Upper Cambrian Obolus sandstone, at Jaggo- 
wal, Esthonia, Russia, in which the heart-shaped cavity is nearly lost, and in which the centrals, 
middle laterals, and outside laterals are crowded into a narrow space. The drawing of the central 
and outside lateral scars is smewhat conventionalized. U.S. Nat. Mus. Cat. No. 35238d. Specimen 
figured by Walcott [1898b, Pl. XX VI, fig. 6]. 
14, 15, 16, and 17. Interiors and posterior portions of dorsal valves from Locality 395b, Upper Cambrian 
Obolus sandstone, at Ilgast, Esthonia, Russia, showing the variation in position of muscle scars. 
U.S. Nat. Mus. Cat. Nos. 51525c-f, respectively. 
OBOLUS APOLLINIS MAXIMUS Mickwitz (p. 384). 
[Pl. XIV, figs. 7 and 7a.] 
Ficure 9. Interior of a ventral valve in which the thickening of the shell is very great. From Locality 395, Upper 
Cambrian Obolus sandstone, at Joa, near Jegelecht, Esthonia, Russia. U.S. Nat. Mus. Cat. No. 
51528a. ; 
28 
