508 CRUSTACEA. 



of these being entirely extinct. In Cypridina (fig. 361, l) the cara- 

 pace is produced in front into a beak-like projection, below which 

 is a hollow or notch facing the ventral margin. Many of the so- 

 called Cypridina of the older Palaeozoic rocks are now known to 

 be referable to other types, but the genus is well represented in the 

 Carboniferous, and exists at the present day. Entomis (fig. 361, 

 j and k) ranges from the Ordovician to the Carboniferous, but 

 attains its maximum in the Devonian rocks. The carapace in this 

 genus resembles that of some of the Leperditidce in having a dorsal 

 groove, indenting the valves transversely, and sometimes reaching 

 the ventral margin, and having a rounded tubercle placed at or near 

 its lower end. One species of this genus {Entomis serratostriata), 

 formerly known as a Cypridina, is so abundant in certain of the 

 Devonian strata of Germany as to have gained for these the name 

 of " Cypridinen-Schiefer." Entomoconchus (fig. 361, m), again, is a 

 large form, with a thick and globose carapace, having a much less 

 developed notch in front than in Cypridina. It is confined to the 

 Carboniferous rocks. Among other Carboniferous Cypridinidce 

 may be mentioned the genera Cyprella, Cypridella, Cypridellina, 

 Sidcnna, and Rhombina, all of which are characteristic of the 

 Carboniferous rocks. 



Another group of the Ostracodes (Poly cop idee) is that character- 

 ised by the genus Poly cope (fig. 361, 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 Devonian and Carboniferous rocks. 



The genus Cytherella is the type of another group (Cyt/zerellida), 

 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 the latter throughout the whole 

 circumference, and " presenting round the entire inner margin a 

 distinct groove, into which the valve of the opposite side is re- 

 ceived " (G. S. Brady). The genus ranges from the Carboniferous 

 to the present day ; and we may provisionally place with it the 

 Cytherellina and sEchmina of the Silurian. 



Lastly, we have the great group of Ostracodes represented by the 

 families of the Cypridce and Cytheridce, " including all the fresh- 

 water and a vast majority of the marine Ostracoda, and embracing 

 all the forms classed by the earlier writers under the two great 

 genera Cypris and Cythere " (G. S. Brady). 



In the Cypridce, as typified by Cypris itself (fig. 361, a and o), 

 the valves are thin and smooth, and more or less sinuate below. 

 Most of the Cypridce are inhabitants of fresh water, and the fossil 

 forms are principally found in lacustrine or fluviatile deposits, often 



