The Galena and Ma<iuoketa Series. — Snrdeson. 99 
Our theory regarding the origin of the double median plication in O. 
ninltisecta appears to fall unsupported in O. ineeki, unless it be as- 
sumed that this one was earlier derived from an O. initltisecta-like 
form, having become larger and more extended in the iiieantime than 
that one. Of the two, O. meeki is the less convex, and has correspond- 
ingly additional plications, especially those between the paired median 
ventral plications being earlier and oftener developed. 
ORTHIS IGNOTA n. sp. 
PI. V, fig. 1 to 7. 
In the Wykoff formation, in bed 1.3, are found in southeastern Minne- 
sota, many shells of a superficial resemblance toothers (Orthis corjnden- 
ta Sardeson), with which they are associated, but which are still so dis- 
tinct that no confusion of the two species is found even in a single spec- 
imen among several thousands examined. It is similar to O. multisecta 
or O. meeki. 
This species is quadrangular rather than circular in outline and is 
broader than long. The cardinal line is always shorter than the width 
<if the shell. The ventral valve is convex while the dorsal valve is 
nearly fiat. On the ventral valve a longitudinal depression on either 
side of the median line bounds the median convexity, but the same may 
be entirely reduced at the anterior margin. On the dorsal valve a cor- 
responding median depression which gradually expands from near the 
beak does not disappear with advanced growth nor does the raised por- 
tion on either side of it. Accompanying this manner of development 
the anterior margin is necessarily straightened or indented. As in most 
"O. testiidinaria'''' seen, a division into five radial surfaces on each valve 
is found, and of these three on the dorsal valve are concave or flat- 
tened and two intermediate ones are convex. The unequal develop- 
ment of these divisions produces the suggestion of quadrangular out- 
line in O. ignota. 
The beak of the ventral valve is curved in towards that of the dorsal 
valve. The latter is straight. The area on the ventral valve is scarcely 
twice as large as on the dorsal and the triangular foramen is nearly 
closed by the cardinal process from the dorsal valve. This cardinal pro- 
cess when well preserved is posteriorly strongly tripartate. and has the 
central part much the most developed. The crural plates reach down- 
ward nearly to the ventral valve. 
The rounded septum between the muscle scars on the dorsal valve is 
smaller than in O. corpulenta and the adductor muscle scars are not 
surrounded by elevated callists but are simply depressed surfaces deep- 
est posteriorly. There is a ridge, however, betw^een the anterior and 
posterior pairs. The anterior pair of adductor scars are much the 
larger. The muscle scars of the ventral valve are conspicuously large, 
and extend far forward. The oblong divaricator scars are close to- 
gether and between and behind them is an elliptical adductor scar, on 
either side of which is a large oval adjustor scar. Close to the pedicle 
attachment scar are two small accessory adductors faintly marked. 
The pedicle attachment is large and distinct. 
