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 be'en led to alter the opinion 

 expressed in 1890, 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 Ceratopsis. This horn-like process is a structural peculiarity, and while 

 it may be analogous or even homologous with the central horn of sEchmina 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 1890 I th&ught 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 peculiarity 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 

 "trisulcate" 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 Bci/ric/iia 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 "trisulc;it;r" 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 (i. e. starting from the dorsal margin). 



But the principal reason for separating these forms from Tetradella is a genea- 

 logical one. The "trisulcate" or " quad ri jugate" lieyricliia, namely, are regarded as 

 a development from the third section of liolflti described on page 668, and which 



