OSTRACODA. 679 
Tetradella 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. trisuleata Hall, and Kladenia 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 from the 
typical trilobed (bisulcate) Beyrichia, nor from Kledenia is perfectly clear to me, 
since it would be necessary to assume a division of the small or middle lobe of those 
genera, which | 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, Bb. ribeiriana 
Jones, B. affinis J., B. bussacensis J., B. lacunata J., B. marchica Krause, B. erratica K., 
B. palmata K., T. signata K., T. carinata K., and T. harpa i. 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 QUADRILIRATA [Hall and Whitfield, and varieties. 
' PLATE XLVI, FIGS. 1—11. 
Beyrichia quadrilirata H. and W., 1875. Pal. Ohio, vol. ii, p. 105. 
Beyrichia regularis MILLER, 1875. Cin. Quart. Jour. Sci., vol. ii, p. 8351. 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. 
S1zE.—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 
