658 THE PALEONTOLOGY OF MINNESOTA. . 



LBeyriohia inltialls. 



Beyrichia initialis, n. sp. 



PLATE XLIII, FIGS. 82 and 83. 



Size.— Length 0. 65 mm.; bight 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 bat 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 BeyHchia 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 Kloedenia, Jones and HoU, 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.— HUAdle third of the Trenton shales, Minneapolis, Minnesota. 



Genus EURYCHILINA, Ulrich. 



Eurychilina, Ulrich, 1889, Oontri. to Can. Micro-Pal., pt. 2, p. 52; also 1890, Jour. Cin. Soc. Nat. 



Hist., vol. xiii, p. 125. 



Carapace with a long, straight binge 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 alhaost to the level of the true or closing edge of the valve; 

 area terminated in most cases by a narrow rim-like border, fiinge simple. Surface 

 beautifully reticulated," pitted, granulose or smooth. 



Type: E. reticulata Ulrich. 



*Jour. Oin. Soo. Nat. Hist., vol. 13, pi. 10, flgs. 5a and 15a; 1800 



