OSTRACODA. 651 
Primitia minutissima.] 
broad frill which projects greatly beyond the free contact edges of the valves in 
species of that genus. As usual, the original conception of Primitia was altogether 
too broad, and as, through the restless efforts of collectors, the species began to 
multiply, it became clear that they fell naturally into several groups, whose import- 
ance increased with time and study till their separation became, at first desirable, 
then necessary. 
Still, Primitia retains a large number of species, the greater part of which are 
nearly equally divided between the Lower and Upper Silurian rocks. Two or three 
rather doubtful species have been described from primordial strata, but at least five 
good Devonian species have been discovered and as many more in the Lower Car- 
boniferous, after which the genusseems to have become extinct. Witha few exceptions 
all these species were described in papers by Jones, Jones and Holl, Krause, and 
Ulrich. 
PRIMITIA MINUTISSIMA, %. Sp. 
PLATE XLV, FIG, 31. 
$1zE.—Length 0.33 mm.; hight 0.19 mm. 
Carapace very small, rather elongate-elliptical in outline, without distinct dorsal 
angles, the ends rounded and nearly equal, the anterior slightly narrower than the 
other; valves rather strongly convex; sulcus narrow, sharply defined, extending 
nearly half across the valve; surface smooth. : 
This is the smallest Primitia known to me. It is evidently related to the British 
Wenloch species, P. humilis Jones and Holl, but is smaller, relatively more convex, 
with the ends more rounded, and the sulcus narrower. Itis not very closely related 
to any of the known American species. 
Formation and locality.—Lower part of the Trenton shales, near Fountain, and at Oxford Mills, 
Goodhue county, Minnesota. 
PRIMITIA UPHAMI, %. Sp. 
PLATE XLIII, FIG. 66. 
Sizzu.—Length 0.42 mm.; hight 0.27 mm.; thickness 0.15 mm. 
Valves small, compressed-convex, slightly oblique, subovate, without distinct 
dorsal angles; posterior end wider and more broadly rounded than the anterior; 
ventral margin convex; edges thin, without border; sulcus represented by a rather 
large, though not very deep depression, situated.about in the middle of the dorsal 
slope; surface marked by small punctz, arranged in curved lines radiating from the 
sulcus; in certain lights each row appears as occupying the bottom of a narrow 
groove, é 
