680 THE PALEONTOLOGY OF MINNESOTA. 
(Tetradella lunatifera. 
ridge has a decided thickening above, and is less distinctly divided below. The two 
posterior ridges also are not entirely distinct. Inthe majority of these lower Trenton 
representatives of the species a delicate ridge or raised line is to be noticed just 
within the posterior portion of the marginal ridge. This is wanting, as far as 
observed, in the Ohio specimens, but in the related T. lunatifera this small ridge is 
represented by one that is quite as strong as the marginal ridge itself. 
Figures 9 to 11 are taken from a variety of which several examples were collected 
at Fountain, Minnesota. These are thicker ventrally than usual (see the last of the 
series of measurements given above), longer, and have an unusually wide flattened 
border, turned outward at the edge. Some slight differences may also be noticed 
in the characters of the median ridges, but the most striking of all their peculiarities 
is the absence of the five marginal cavities. In some respects these specimens agree 
very well with the var. simplex described by the author from Hudson River shales 
in Manitoba, but as they are not identical another subordinate name might appro- 
priately be applied to them. 
Formation and locality.—Birdseye limestone, High Bridge, Kentucky; middle and upper third of 
the Trenton shales, Minneapolis, St. Paul, Cannon Falls, Fountain, and other localities in Minnesota; 
upper beds of the Cincinnati group at Clarksville, Blanchester, Waynesville and Oxford, in Ohio, Rich- 
mond and Versailles in Indiana. 
TETRADELLA LUNATIFERA Ulrich. 
PLATE XLVI, FIGS. 12—14. 
Strepula lunatifera ULRICH, 1889. Contri. to Can. Micro.-Pal.. ii, p. 56. 
Tetradella lunatifera ULRICH, 1890. Jour. Cin. Soc. Nat. Hist., vol. xiii, p. 112. 
Fig. 51.—Two valves of 7’. lunatifera from the Galena shales near Cannon Falls, x22; showing differ- 
ences in the ridges. 
Sizz.—Length 1.28 mm.; hight 0.75 mm.; thickness 0.58 mm. 
This species is in a general way much like 7. quadrilirata but differs more or less 
obviously from that, as well as from all other species now referred to the genus, in 
having in all six ridges instead of the usual four. Two of this number however were 
produced by division of the posterior and antero-median ridges. All four of the 
inner ridges may be, as shown in the above cut, separate except at their lower ends 
where they unite with the marginal ridge. In others (see plate xuv1, fig. 12) the 
antero-median pair may be so near each other as to form practically but a single 
ridge. In others again this pair is united above and below but bent in such a 
manner that they enclose a crescent-shaped hollow space. Finally, in a few cases 
