„ ^ ^ OSTRACODA. 677 



Tetradella.l 



Variety robusta, n. var. 



Beyriohia chambersi Hall and Whitfield, Pal. Ohio, vol. li, p. 104, pi. 4, flgs. 11, 12, not strictly B. 



ehambersi 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 desigDation 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 Ehynchotreta 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 Gannon Tails. As yet, it has not been 

 detected in any of the divisions of the Galena, out 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 TETEADELLA Ulrich. 



Tetradella (part.) Ulkich, 1890. Jour. Cin. Soc. Nat. Hist. vol. xiii, p. 112. 



Strepula, Ulbich, 1889. Contrl. to Can. Micro.-Pal.. pt. 2, pp. 54, 56, not of Jones and Holl. " Tri- 

 sulcate " and " quadri jugate " 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 (T. subquadrata) thick and with the contact margins 

 concealed by a "false border." Surface smooth or granulose. 



Type: T. (Beyrichia) quadrilirata Hall and Whitfield. 



