OSTRACODA. 649 
Priroitiella fillmorensis.] 
PRIMITIELLA FILLMORENSIS, 2. sp. 
PLATE XLV, FIGS. 28-30. 
Sizz.—Length 0.55 mm.; higbt 0.38 mm.; thickness 0.23 mm. 
A small, comparatively short form, with subequal, rounded ends, broadly curved 
ventral margin and a straight back, the extremities of which however are scarcely 
angular; mesial depression very shallow but wide, taking up a large portion of the 
centro-dorsal region; edges simple, or with an obscurely defined, narrow border, 
Though resembling several species of Primitia, from which it is distinguished by the 
width and shallowness of the mesial depression, I cannot find any described ostracode 
with which it is strictly identical. 
Formation and locality.—Not uncommon in the lower part of the Trenton shales near Fountain, 
Minnesota. 
PrimitiELLA untcornis Ulrich. 
PLATE XLIII, FIGS. %5—77. 
Leperditia unicornis ULRICH, 1879. Jour. Cin. Soc. Nat. Hist., vol. ii, p. 10, pl. vii, fig. 4. 
?Aparchiles unicornis ULRICH, 1889. Contri. Can. Micro-Pal., pt. 2, p. 50. 
2Primitia unicornis JONES, 1890. Quart. Jour. Geol. Soc., vol. xlvi, p. 7. 
Si1zz.—Length 1.04.; hight 0.59 mm.; thickness 0.4 mm. 
Carapace convex, scarcely oblique, oblong, with a long, straight hinge, and 
rather well-marked dorsal angles; ventral margin gently arcuate, nearly parallel 
with the back, the anterior hight of the valves being but little less than the poste- 
rior; posterior margin neatly rounded from the dorsal angle; anterior margin obliquely 
truncated in the upper half, sharply rounded at the middle; a narrow but well- 
defined border begins at this point and follows the outline to the post-dorsal angle; 
near the posterior extremity of each valve, usually somewhat beneath the mid-hight, 
a strong spine projects outwardly or posteriorly; in a dorsal or ventral profile the 
ends are blunt and the sides of the valves straight or just appreciably concave; in 
front of the middle of the dorsal slope there is a wide and very faint depression, and 
in the lower part of this a low swelling is almost invariably distinguishable. Speci- 
mens are usually about 1.0 mm. in length. A small variety occurs at Cincinnati, 
Ohio, averaging between 0.5 and 0.6 mm. in length. 
All the American specimens of this species, excepting the valve figured by me 
from Manitoba (op. cit.) are remarkably constant in all their characters. Indeed, out 
of over fifty free carapaces and valves, I was unable to find one that differed enough 
from fig. 77 to make it worth the while to prepare drawings of it. Bearing this 
constancy in mind it is rather surprising to learn that Prof. Jones found considerable 
variability among the British specimens referred by him to the species. Comparing 
