OSTRACODA. 677 
Tetradella.] 
Variety ROBUSTA, 7”. var. 
Beyrichia chambersi HALL and WHITFIELD, Pal. Ohio, vol. ii, p. 104, pl. 4, figs. 11, 12, not strictly B. 
chambersi Miller. 
Fig. 50.—Lateral and posterior views of an entire carapace of this variety from the shales of the 
Hudson River group, near Spring Valley, Minnesota. 
This designation is proposed for the variety which occurs in the upper beds of 
the Cincinnati group at numerous localities in Ohio, Indiana, and Kentucky, and in 
the equivalent Hudson River group strata of Minnesota. So far as known it is not 
to be found below the horizon of Orthis subquadrata Hall, and Rhynchotreta capax 
Conrad. It differs from the typical form of the species in having all the ridges some- 
what thicker, and the post-median one much larger. In many cases the latter is 
nearly or quite equal to the anterior ridge, and extends like it entirely across the 
valve. ‘The ventral portion of the carapace also is thicker, and the marginal ridge 
subangular where the contour turns abruptly inward to the false border. 
Formation and locality.—The typical form of the species is rare in the middle and common in the 
upper third of the Trenton shales at Minneapolis, St. Paul and Cannon Falls. As yet, it has not been 
detected in any of the divisions of the Galena, nut in the lower beds of the Cincinnati group it is a com- 
mon fossil. The var. robusta was found in the upper part of the Hudson River shales near Spring Valley, 
Minnesota, and occurs abundantly in the upper beds of the Cincinnati group at Waynesville and Oxford, 
Ohio, Richmond and Versailles, Indiana, and many other localities in these states. 
Genus TETRADELLA Ulrich. 
Tetradella (part.) ULRICH, 1890. Jour. Cin. Soc. Nat. Hist. vol. xiii, p. 112. 
Strepula, ULRICH, 1889. Contri. to Can. Micro.-Pal.. pt. 2, pp. 54, 56, not of Jones and Holl. “ Tri- 
suleate” and ‘‘ quadrijugate” Beyrichia (part.) of authors. 
Carapace somewhat oblong, often subquadrate, never tumid, with the hinge line 
straight. Surface depressed, with a semicircular marginal ridge; within the enclosed 
space, two, simple or slightly modified, equal or unequal, and more or less nearly 
vertical ridges unite below with the marginal ridge and extend upward from it, one 
‘In many cases failing to reach the dorsal margin. Free edges usually with a simple 
flattened border; in one case (7. subquadrata) thick and with the contact margins 
concealed by a “false border.” Surface smooth or granulose. ; 
Type: T. (Beyrichia) quadrilirata Hall and Whitfield. 
