OSTRACODA. 631 
Ostracoda. ] 
edges of the valves. The genus Hurychilina affords excellent examples of species 
with a “frill.” 
As regards the geological or time distribution of the Ostracoda, it is certain that 
they began in the upper divisions of the Taconic system, if indeed they are not to 
be reckoned among the earliest fossils known. In the Lower Silurian deposits already 
they occur in such great numbers and variety, that it is doubtful if the representa- 
tions of the order at any subsequent time exceeded them in these respects. The 
predominant types, Leperditiide and Beyrichiidw, moreover, while holding their own 
perhaps through the Upper Silurian, were greatly reduced during Devonian and 
Carboniferous times and are now totally extinct. Some recent families and genera 
on the other hand were sparingly represented, but taken as a whole the Silurian 
Ostracoda fauna is decidedly peculiar.* 
In the Upper Silurian formations the Leperditiude and Beyrichiide still predom- 
inated, but the fauna here received decided accessions in the way of genera regarded 
as belonging to the family Cypride. The Devonian Ostracoda, though less numerous, 
are not very different from the Upper Silurian types, most of the old genera being 
more or less sparingly represented. Several genera (¢. g. Kyammodes, Jones, and 
Barychilina, Ulrich) are so far to be considered as peculiar to this system of rocks. 
The Ostracoda fauna of the Carboniferous deposits, on the contrary, while retaining 
many small species of essentially Silurian genera like Leperditia, Beyrichia and 
Primitia, which occur associated with the related genera Beyrichiella and Beyrichi- 
opsis and numerous forms of the previously established types of the Cypride, 
nevertheless assumed a distinctive aspect through the strong development of hitherto 
unknown types of Cyprinide. 
In succeeding formations the. Ostracoda are everywhere poorly represented in 
the Triassic and Jurassic. But in the Cretaceous and Tertiary strata of Europe cer- 
tain genera, Cythere especially, develop an astounding variety and wealth of species. 
The forms are all small, and this may in part account for the fact that so few have 
been discovered in American deposits of these ages. 
The recent genera having, or believed to have, paleozoic representatives, occur 
in the various formations as follows: Cypridina, Bradycinetus and Philomedes, in 
the Carboniferous; Polycope, Silurian and Carboniferous; Cytherella and Cythere, 
’ Silurian, Carboniferous and Permian; Cythereis and Cytherideis, Permian; Bairdia, 
Silurian, Devonian, Carboniferous and Permian; Bythocypris, Silurian, Devonian, 
Carboniferous; Macrocypris, Silurian and Carboniferous; Pontocypris, Silurian; and 
Aglaia, Argillecia and Candona, in the Carboniferous. 
*That the Silurian species which are now placed into recent genera uctually belong there may well be questioned. 
In my opinion they do not, yet, as they cannot, with our limited opportunity for comparison, be distinguished, I am obliged 
to agree that the aims of classification ure for the time being sufficiently satisfied, 
