680 THE PALEONTOLOGY OF MINNESOTA. 



LTetradella lunatlfera. 



ridge has a decided thickening above, and is less distinctly divided below. The two 

 posterior ridges also are not entirely distinct. In the 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. lunatlfera this small ridge is 

 represented by one that is quite as strong as the marginal ridge itself. 



Figures 9 to 1 1 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. 



TETRABELLA LUNATIFERA Ulrich. 



PLATE XLVl, FIGS. 12-14. 



Strepula lunatifera Ur.iucn, 1889. Contri. to Can. Micro.-Pal.- ii, p. 56. 

 Tetradella lunatifera ULKICH, 1890. Jour. Cin. Soc. Nat. Hist., vol. xiii, p. 112. 



l-'ig. 51. Two valves of T. lunatifera from the Galena shales near Cannon Falls, x22; showing differ- 

 ences in the ridges. 



-Length 1.-.J* mm.; night 0.75 mm.; thickness 0.58 mm. 



Tliis species is in a general way much like T. 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. Twoof this number however were 

 produced by division of the posterior and antero-median ridges. All four of the 

 inner ridges may he, as shown in the above cut, separate except at their lower ends 

 where they unite with the marginal ridge. In others (see plate XLVI, fig. 12) the 

 antero-median pair may l)e 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 



