678 THE PALEONTOLOGY OF MINNESOTA. 
(Tetradella. 
In the original definition of this genus (loc. cit.) I included as a section the 
species that I now separate as Ceratopsis, under which name they have been distin- 
guished in my private collection since 1881. I have been led to alter the opinion 
expressed in i890, respecting the desirability of generically recognizing the 
distinguishing peculiarity of Ceratopsis by repeated comparison among the constantly 
increasing typical species of the genus. Of the fifteen good, and four somewhat 
doubtful species of Tetradella now known, not one shows the remotest sign of the 
“horns” of Ceratupsis. This horn-like process is a structural peculiarity, and while 
it may be analogous or even homologous with the central hora of Aichmina and the 
two horns of Dicranella, it is more highly organized, and surely deserves generic 
recognition when this rank is accorded to the more simple process in the two cases 
mentioned. In 1590 I thought it just possible that the feature might prove incon- 
stant, if not abnormal, but that is now quite out of the question since it is as constant 
as any pecuilarity can be, being repeated in thousands of examples of each of the 
three American species, during unusually long geological ranges, and with a persist- 
ency of specific marking that would be most extraordinary if the feature was not 
of structural importance. 
The affinities of Tetradella seem to be with Ctenobolbina on the one hand and the 
“trisuleate” species of Beyrichia, which as I have shown on page 668, are generically 
distinct from Beyrichia and provisionally to be viewed as a section of Bollia, on the 
other. In the former, however, there are only two or three ridges instead of four, 
the space occupied by the two posterior ridges in Tetradella being representedby a 
single large bulb. The valves also are more convex, especially when, as is generally 
the case, the anterior sulcus is wanting or but feebly developed, and the free edges 
are thicker, while the “false border,’ which is almost unknown in the present 
genus, is generally well developed in Ctenobolbina. 
The resemblance to the trisulcate Beyrichia is more marked and may prove 
troublesome to those who have not made a special study of the Ostracoda. Still, I 
remember no case now, in which one more or less well marked difference cannot be 
made out. Namely, in the “trisulcate” the arrangement of the sulci and ridges is 
approximately symmetrical and bilateral, the central sulcus being vertical, while 
the two lateral sulci curve outwardly. In Tetradella, however, this symmetrical 
arrangement is not evident since it is generally the case that all the sulci curve 
more or less posteriorly (¢. e. starting from the dorsal margin). 
But the principal reason for separating these forms from Tetradella is a genea- 
logical one. The “trisulcate” or “quadrijugate” Beyrichia, namely, are regarded as 
a development from the third section of Bollia described on page 668, and which 
