658 THE PALEONTOLOGY OF MINNESOTA. 
[Beyrichia initialis. 
BrYRICHIA INITIALIS, 2. Sp. 
PLATE XLIII, FIGS. 82 and 83. 
Size.—Length 0.65 mm.; hight 0.41 mm.: thickness 0.30 mm. 
Valves small, somewhat oblong, subquadrate; hinge line straight, nearly as long 
as the valve; dorsal angles distinct without being sharp; ventral margin but little 
convex, nearly parallel with the back; ends subequal, neither much curved; free 
margins with a distinct border or flange, turned outward. Middle lobe situated just 
above and a little in front of the center, rather low, rounded, not sharply separated 
from the anterior lobe; mesial sulcus deeper than the anterior, meeting beneath the 
small lobe; anterior*lobe rather small, coalescing ventrally with the much larger 
posterior lobe, the junction faintly indicated. In the anterior part of the valve the 
surface is depressed, but in the upper corner a small tubercle is to be noticed. 
In this species the isolation of the small lobe has progressed beyond the limits 
of Primitia, and the result is sufficiently close to Beyrichia to be included in this 
genus. An approach toward Beyrichian characters is faintly indicated in Primitia 
duplicata and P.tumidula, while the tendency to vary in this direction is much better 
expressed in certain varieties of P. cincinnatiensis Miller, and P.? parallela Ulrich.* 
The latter might be called a Kledenia, Jones and Holl, a genus that, with slight 
peculiarities of its own, seems to be nothing more than a recognition of one of the | 
* more permanent transitional types between Primitia and Beyrichia. 
Specifically, B. initialis is not likely to be confounded with any Lower Silurian 
ostracode known to me. Nor is there any pressing need of comparing it with its 
much larger Upper Silurian congeners. 
Formation and locality.—Middle third of the Trenton shales, Minneapolis, Minnesota. 
Genus EURYCHILINA, Ulrich. : 
Hurychilina, ULRiIcH, 1889, Contri. to Can. Micro-Pal., pt. 2, p. 52; also 1890, Jour. Cin. Soc. Nat. 
Hist., vol. xiii, p. 125. 
Carapace with a long, straight hinge-line; semicircular, oblong-subquadrate, or 
somewhat rounded in outline; generally with a well-defined subcentral vertical 
sulcus and a more or less prominent node immediately behind -it. Except at the 
dorsal side, the valves are surrounded by a wide marginal area, externally either flat 
or convex and usually marked in a radial manner; on the inner side deeply concave, 
an outer wall being raised almost to the level of the true or closing edge of the valve; 
area terminated in most cases by a narrow rim-like border. Hinge simple. Surface 
beautifully reticulated, pitted, granulose or smooth. 
Type: L. reticulata Ulrich. 
‘Jour, Cin, Soe, Nat, Hist., vol. 13, pl. 10, figs. 5a and 15a; 1890 
