The Galena and 3faquoketa Series. — Sardeson. 95 
It requires a large collection to aid in distinguishing specif- 
ic or varietal characters from the individual peculiarities, al- 
though there is an additional assistance in certain abnormal 
or occasional specimens which present instead of the two nor- 
mally symmetrical parts on the right and left of the median 
plane, two extremes of development nearly equal to the differ- 
ences in any two separate individuals of the variety or species. 
And that is true of both the internal and the external features 
of the shells. 
Ortfiis testudinaria auct. includes the following species : — 
ORTHIS ROGATA Sardeson. 
PI. IV, fig. 1 to 10. 
Orthis rogata, Sardeson, (Apr. 9, 1892). Extr. Bull. Minnesota Acade- 
my Natural Sciences, vol. iii, p. 3.31, pi. 5, fig. 1 to 4. 
Orthis testudinaria Hall, (1892). Pal. N. York, vol. viii. pt. 1, pi. 5 B, 
fig. 27 to 31. 
Orthis (Dalinanella) hamburgensis W. and S. (1893). Final Rep. Ge- 
ol. Nat. Hist. Sur. Minnesota, vol. in, p. 440, pi. 33, fig. 14 to 16. 
Orthis {Dal.) testudinaria. W. and S. Loe. cit. p. 441, pi. .33, fig. 17 
to 22. 
This shell is found in the Upper Mississippi valley in the Ordovician 
strata, ranging from the top of the Stictopora bed (4) to the Lingulelas- 
ma h>ed (8). and is very abundant in the Fucoid (5) and Orthisina (6) beds. 
It is small and has a subcircular outline, the hinge line two-thirds to 
three-fourths the greatest width of the shell, and the cardinal angles 
broadly rounded. The ventral (pedicle) valve is convex and the dorsal 
(brachial) valve nearly plane, but the convexity of both is often much 
beyond the normal condition. Aged specimens generally are very strong- 
ly and irregularly convex near the margin of the valves and dorso vent- 
ral growth at the shell margins may mark the final stage of shell-build- 
ing. In such cases, concentric lines, which seem to be only from inter- 
rupted growth of the animal, are numerous. Concentric lines occur at 
any stage of growth of the shells however. Dwarfed specimens show 
great convexity, as a rule. Spiniform cardinal angles are an accidental 
character. On young specimens the ventral valve has a median ridge 
or fold and the dorsal a corresponding depression or sinus, but both and 
particularly the latter vanish toward the margin in the mature grown 
shells. Both the acutely curved beak of the ventral valve and the ob- 
tuse and nearly straight one of the dorsal have been shortened by etch- 
ing or erosion on the senile shells. The cardinal area of the ventral valve 
is about three times as high as that of the dorsal. The cardinal process 
is small, but variable in length. A strong rounded sejjtum extends be- 
tween the dorsal adductor scars to a little forward of the same. The 
scars are strongly marked, the anterior pair being the larger. Some 
greatly thickened shells have ridges extending obliquely forward from 
the crura, and thickening along the vascular spaces, both of which tend 
