OSTRACODA. 643 
Sechmidtella subrotunda.] 
SCHMIDTELLA SUBROTUNDA, 7%. Sp. 
PLATE XLY, FIGS. 39—42. 
Si1zE.—Length 0.5 mm.; hight 0:43 mm.; thickness 0.3 mm. 
Valves small, short, rounded-ovate, rather uniformly convex, with an obscurely 
defined, narrow border around the ends and ventral margin; near the center a faint 
depression, and immediately behind it a small elevation. 
The generic position of this small species is uncertain. It is placed under 
Schmidtella chiefly because it seems to be related to S. wmbonata, though much 
shorter. Its outline is almost exactly as in the Cytherella ? subrotunda of this report, 
which was also found associated with it. Possibly they belong to one species. Still, 
as the type of the Cytherella has neither a central pit nora border, I am for the 
present obliged to regard them as distinct. 
Formation and locality—Lower third of the Trenton shales, Minneapolis, Minnesota. 
Genus APARCHITES, Jones. 
Aparchites, JONES, 1889. Ann. and Mag. Nat. Hist., ser. 6, vol. iii, p. 385. 
Carapace subovate, oblong, or somewhat rounded, with a straight hinge of 
variable length; valves subequal; edges thickened, never overlapping, often beveled 
or chaneled, in other cases simple, and rarely with a narrow flattened border. Surface 
more or less convex, usually smooth, without sulcus, tubercles or lobes. 
Type: A. whiteavesii Jones. 
The above definition embraces a number of species that had formerly been 
placed under Primitia, Isochilina and Leperditia. From the first they are distin- 
guished by the absence of a sulcus, from the second by the absence of the 
eye-tubercle and certain shallow depressions behind it, and from the third by the 
absence of the eye-tubercle and the equality of their valves, there being no ventral 
overlap. From Leperditella they are separated by their equal valves, the left 
overlapping the right inthat new genus. Finally, the new genus Primitiella includes 
some very similar carapaces, but these may be distinguished, in most cases very 
easily, by a broad though quite undefined depression or sulcus in the centro-dorsal 
region. 
The species of Aparchites are all small, the average length being between 1.0 
mm. and 1.5 mm., while the largest known does not exceed 3.0 mm. The total 
number of those known, including several undescribed species from Ohio, probably 
exceeds twenty. These are distributed almost equally between the Lower and 
Upper Silurian rocks, though in America they are known chiefly from the Trenton 
and Cincinnati formations. 
