The Galena and JIaquoketa Series. — Sardefiou. 97 
the median plication normally has paired offsets instead of alternating 
ones like the other primary plications, and again after the exact 
bilateral symmetry has been disturbed to an alternating arrangement 
by dwarfing; the subsequent pairing of the median with its first branch 
may be explained as restoration of bilateral symmetry merely. Not al- 
ways has dwarfing of the shells disturVjed bilateral symmetry of the pli- 
cation, but as a rule it has, and the retardation of the Vjranching is 
manifest now upon the right or again upon the left side of the median 
plication . 
As will be seen later the variation in O. rogafa Sardeson is in the di- 
rection from the characters seen in O. einaeerata Hall towards those 
seen in O. mwltisecta Meek of the Hudson epoch. 
ORTHIS IVIULTISECTA Meek. 
PI. IV, fig. 20 to 23. 
Orthh emacei-ata var. multisecta (James) Meek (1873). Pateontology 
of Ohio, vol. I, p. 112, pi. 8, fig. 3a, b, c, d. 
This species should not be called a variety of O. emacerata Hall. It 
is the type of a very diffei'ent variety of 'O. tentndiriaria' from that one. 
It is found associated in Cincinnati, Ohio, with both O. emacerata Hall 
and O. iiieeki S. A. Miller, to the latter of which it is the more nearly 
related. 
O. viidtlseeta is a little larger than O. rogata, is often as finely 
striated, but has a pair of primary median plications instead of one on 
the ventral (pedicle) valve; and the median fascicle of plications is there- 
fore divided into two. The plications branch about three times be- 
tween the beak and the margins, but in the median a pair of fascicles 
sometimes only twice, while again one or both median plications may 
branch into the median furrow and there may thus be from 4 to 9 plica- 
tions in one- or both of the median fascicles, the number and fineness of 
them varying in that manner. 
The shell is round anteriorly and laterally, but the position of the 
hinge is such that the shell's length is less than its breadth. The car- 
dinal length varies from one-half to two-thirds the width of the shell. 
The ventral valve is deep and has the convexity greatest near the med- 
ian line, while the dorsal valve is nearly flat with a deep, broad median 
depression expanding and deepening from the beak to the margin. The 
shell is thin unless thickened with ag3. The shell structure and the 
internal characters are nearly like those of Ortliis rogata. 
The relationship between O. rogata and O. midtisecta is expressed no 
doubt in the very great similarity between them. The former is the 
predecessor and could perhaps be the parent species, since it is found 
in beds 1 to 7, while O. midtisecta. is found rarely in bed 12 in Iowa, 
and very abundantly at an equivalent horizon and higher in the Cin- 
cinnati group of Ohio. But it is the dwarfish, convex, small individuals 
of O. rogata that verge toward O. midtisecta, notwithstanding that 
the latter species is the larger, and we find that it is the relative pro- 
portion between the mesial periphery and the convexity of the shell, or 
the same expressed in the manner of plication, in which the agreement 
