OSTRACODA. 643 



Sohmidtella subrotunda.1 



SCHMIDTELLA SUBROTUNDA, n. Sp. 

 PLATE XLV, FIGS. 39-42. 



Size.— Length 0.5 mm.; hlght 0.43 mm.; thickness 0.3 mm. 



Valves small, short, rounded-ovate, rather uniformly convex, vpith 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. umbonata, 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 nor a border, I am for the 

 present obliged to regard them as distinct. 



Formation and looality.— Lower third of the Trenton shales, Minneapolis, Minnesota. 



Genus APAROHITES, Jones. 



Aparchites, Jones, 1889. Ann. and Mag. Nat. Hist., spj. 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 in that 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. 



