OSTRACODA. 679 



Teferadella quadrilirata.] 



includes B. unguloidea, B. persulcata, B. regularis, etc. A good demonstration of this 

 line of development may be established already from known species. Compare, for 

 instance, B. regularis Emmons sp., Lower Silurian, and Beyrichia clarki Jones, B. 

 halli, Jones, B. hieroglyphica Krause, B. trisulcata Hall, and Kloedenia kiesowi Krause, 

 Upper Silurian, and it is clear that the change from the first to the last was nothing 

 more than a gradual coalescence of the ventral curves of the inner and outer ridges 

 and the consequent obsolescence of the sulci. 



That B. trisulcata and similar forms could not have been developed fi;om the 

 typical trilobed (bisulcate) Beyrichia, nor from Kloedenia is perfectly clear to me, 

 since it would be necessary to assume a division of the small or middle lobe of those 

 genera, which I think I am safe in declaring, never took place. 



Tetradella is essentially a Lower Silurian genus, nearly all the typical species 

 being restricted to strata belonging to, or equivalent to the Trenton and Cincinnati 

 formations. In America we have T. quadrilirata Hall and Whitfield, and var. simplex 

 Ulrich, T. lunatifera and T. subquadrata Ulrich. Of European species doubtlessly 

 belonging to Tetradella I may mention Beyrichia complicata Salter, B. ribeiriana 

 Jones, B. affinis J., B. bussacensis J., B. lacunata J,, B. marchica Krause, B. erraticdK., 

 B.palmata K., T. signata K., T. carinata K., and T. harpaK. As somewhat doubtful 

 Upper Silurian representatives, we may regard four species figured by Dr. Krause, 

 viz.: Beyrichia digitata K., B. dissecta K., B. mamillosa K., and B. nodulosa Boll. In 

 the first the ridges do not appear to unite ventrally, and in the last the anterior pair 

 are peculiarly twisted together, while in the second and third all the ridges are 

 divided into nodes, two nodes taking the place of each ridge. 



Tetradella quadrilibata Hall and Whitfield, and varieties. 



PLATE XLVI, FIGS. 1-11. 



Beyrichia quadrilirata H. and W., 1875. Pal. Ohio, vol. ii, p. 105. 



Beyrichia regularis Mji.T,^Tt,, ISIb. Cin. Quart. Jour. Sci., vol. il, p. 351. Not B. regularis 'Emmons 



Strepula quadrilirata Ulrich, 1889. Contri, to Can. Micro. Pal., pt. ii, p. 54. 



Tetradella quadrilirata Ulrich, 1890. Jour. Cin. Soc. Nat. Hist., vol. xiii, p. 122. 



Size.— Length 1.10 mm,; hight 0.75 mm.; thickness 0.42 mm. 

 Length 0.94 mm.; hight 0.62 mm.; thickness 0.38 mm. 

 Length 1.13 mm.; hight 0.70 mm.; thickness 0.55 mm. 



Figures 1 to 3 are taken from a representative specimen of the species as .it 

 occurs in the Trenton shales of Minnesota. It is also very nearly identical with the 

 typical form which is found so abundantly in the upper beds of the Cincinnati group 

 in Ohio and Indiana. The original of fig. 4 is from the Birdseye limestone at High 

 Bridge, Kentucky. This is somewhat shorter and more oblique than usual. Figure 

 7 represents a variety, not uncommon at Minneapolis, in which the antero-median 



