OSTRACODA. 631 



Ostracoda.] 



edges of the valves. The genus Eurychilina 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, Leperditiidce and Beyrkhiidce, 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 Leperditiidce and Beyrichiidce still predom- 

 inated, but the fauna here received decided accessions in the way of genera regarded 

 as belonging to the family Cypridce. 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 (e. 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 Cypridce, 

 nevertheless assumed a distinctive aspect through the strong development of hitherto 

 unknown types of Gyprinidce. 



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, Brady cinetus 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 VermidM] Bythocypris, Silurian, Devonian, 

 Carboniferous; Macrocypris, Silurian and Carboniferous; Pontocypris, Silurian; and 

 Aglaia, Argillmcia and Candona, in the Carboniferous. 



*That the Silurian species which are now placed into recent genera actually belong there may well he questioned. 

 In my opinion they do not, yet, as they cannot, with our limited opportunity for comparison, he distinguished, I am obliged 

 to agree that the aims of classlflcation are for the time being sufficiently satisfied. 



