Tke Galena and Maquuketa Series. — Sardeson. 101 
and beaks of the shell under adverse surroundings, during growth. 
But other shells in the same strata are not perceptibly injured, and the 
characters in O. tersa are so uniform that the heavy articulating pro- 
cesses and the thickened shell seem to be more like inherited adapta- 
tions than acquired ones, whatever their origin. 
This and the three preceding species may have Vjeen derived from a 
common parent species. They have in common the single dorsal and 
paired ventral median plications, and the surface ornamentation so well 
known in Orthis vinltisecta Meek. The last named character they have 
in common with O. rogata Sardeson, and O. curjjulenta Sardeson. 
From O. rogata they may have inherited the same and thence also de- 
rived their plication. 
ORTHIS CORPULENTA Sardeson. 
PI. IV. tig. 11 to 19. 
Orthis corpulenta Sardeson (1892) Bull. Minnesota Academy Nat. 
Sci. vol. Ill, p. 330, pi. 5, fig. 8, 9, 10. 
Orthis (Dalmanetla) te^tiidinaria var ineeki Winchell and Schuchert 
(189,3), Final Rep. Geol. Nat. Hist. Sur. Minnesota, vol. in, p. 445, pi. 23. 
fig. 25-29. (non S. A. Miller). 
Orthis (Dalmanella) ineeki Hall (1893). Palseontology N. York. vol. 8, 
pt. 1, pi. 5, O. fig. 3. 
The types of the species are from the Lepta3na bed (13) but it is 
equally abundant in the Triplecia bed (10). The shells are nearly cir- 
cular in outline, their width exceeding their length always in young 
specimens but rarely in old ones. The ventral valve is strongly convex, 
the dorsal moderately so, but their convexity is often irregularly in- 
creased in later growth, and the strongly, marked median ridge of the 
ventral and the corresponding depression of the dorsal valves of early 
growth broaden and flatten. The ventral valve has the beak sharjj 
and curved well down to the mesial plane, over the equilaterally trian- 
gular foramen, which same is nearly blocked by the protruding cardinal 
yjrocess. The septum of the dorsal valve is like that of O. rof/afa ex 
cept that it has an elevation between the anterior adductor scars. The 
anterior pair of adductor scars are much larger than the posterior ones, 
and all are often surrounded by thickened lines and vascular furrows 
on the shell which much modifies their appearance. 
The plication is essentially like that of Orthis rogata Sar., with the 
difference, however, that more plications end on the cardinal margins, 
and that hence the two posterior fascicles are quite reduced to sin- 
gle primary plications, not noticeable longer on mature shells. The sur- 
face ornamentation is like on O. innltisecta. Meek. The earliest speci- 
mens of O. corpulenta are from the base of the Triplecia bed (10), and 
the manner of plication, increased convexity, heavy concentric growth 
interruption lines, and small size, prove them to be dwarfed, like the 
dwarfed ones of Orthis rogata Sardeson, which they in all respects par- 
allel. Alternating median offset jjlications, and nearly paired median 
plication are not rare. Nearly all shells have been crushed, apparently 
by the trilobites, which accounts for the small size of some of them. 
