346 ARTHROPODA. 



be mentioned the genera Cyprella, Cypridella, Cypridellina, 

 Sulcana, and Rhoiribina. 



Another group of the Ostracodes is that characterised 

 by the genus Polycope (fig. 201, N), in which the carapace- 

 valves are subequal and thin, not markedly notched in front, 

 and having no beak. Though represented by living species, 

 the only undoubted members of this genus which have been 

 detected in a fossil state are from the Carboniferous rocks. 



The genus Cytherella is the type of another group, in 

 which the valves are very thick and calcareous, and are not 

 notched in front. In Cytherella itself the right valve is 

 much larger than the left, overlapping throughout the whole 

 circumference, and "presenting round the entire inner margin 

 a distinct groove, into which the valve of the opposite side 

 is received " (G. S. Brady). The genus ranges from the 

 Carboniferous to the present day ; and we may provisionally 

 place with it the Cytlierellina and ^Echmina of the Silurian. 



Lastly, we have the great group of Ostracodes represented 

 by the families of the Cypridoc and Cytheridm, " including 

 all the fresh-water and a vast majority of the marine Ostra- 

 coda, and embracing all the forms classed by the earlier 

 writers under the two great genera Cypris and Cythere " (G. 

 S. Brady). In the Cypridw, as typified by Cypris (fig. 201, 

 o) itself, the valves are thin and smooth, and more or less 

 sinuate below. The family seems to have attained its maxi- 

 mum at the present day, but it is represented in deposits as 

 old as the .Carboniferous by forms which are believed to be 

 referable to the living genera Candona and Bairdia. In the 

 Cythcridce the carapace is hard and calcareous, generally 

 with a rough or uneven surface. This family, also, is mainly 

 Eecent, Tertiary, and Secondary ; but it is represented in 

 the Palseozoic period by the living genus Cythere, and by the 

 extinct Thlipsura. 



ORDER PHYLLOPODA. 



Crustacea, mostly of small size, the carapace protecting the 

 head and thorax, or the ~body entirely naked. Feet numerous, 

 never less than eight pairs, mostly foliaceous or leaf -like, bran- 

 chial in function. 



