THE TERTIARY FORMATION. 23 



they even recognise the same forms for Miiller's species, except in the case of C. gihha 

 and C. gibbera. Liljeborg figures as C. viridis of Miiller what Zenker figures as 

 Miiller's C. Jlavida ; and Zenker figures as C. viridis a form that approximates to 

 Liljeborg's C. nitida. Liljeborg's view of C. viridis appears to me to be correct : 

 Zenker's C. Jlavida should be referred to the same species; Miiller's C. Jlavida being a 

 different form. 



S. Fischer has lately ('Munich Transactions/ 1855) further illustrated one of 

 of Miiller's species, and two new forms. 



Genus — Cythere,^ Midler. 



Animal creeping ; chiefly marine.^ Eye single, the coalescence of which is more or 

 less imperfect : upper antennae setiferous, but not presenting a filamentous plume ; 

 lower antennae hooked and furnished with a single, long, jointed filament; feet three 

 pairs. The carapace-valves are usually ornamented on their anterior and posterior 

 borders with a marginal series of fine spines or denticulations, which are coarsest in 

 the sub-genus Cythereis and finest in Bairdia. 



Cythere proper. 



Carujiace often very convex, especially on the ventral portion ; sometimes smooth 

 and setigerous, generally pitted, and occasionally reticulated : varying in outline from 

 an acute-oval to an irregular oblong ; in the first case it often resembles a peach-stone 

 in miniature ; in the latter case a central and two posterior tubercles sometimes give a 

 character to the valves -^ and in each case the anterior, and sometimes the posterior, 

 hinge forms an indistinct angle on the dorsal edge; the hinge-line of each valve 

 occupies about the middle third of the dorsal margin, and presents a ridge or bar and 

 a furrow, the bar on one valve corresponding to the furrow on the other ; the bar is 

 sometimes blended with the edge of the valve, and is occasionally finely crenulated ; 



i For synonyms, see ' Monog. Entom. Cret., 1849, p. 8. 



2 Cythere inopinata, Baird, and another species referred to by Say, are the exceptional species in this 

 respect. According to Zenker, cue species, at least, of marine Cytheres can live for several days in fresh 

 water ; and, on the other hand, S. Fischer describes a Cypris from saltish water at the mouth of the Neva, 

 and another from the sea-water of the harbour at Alexandria. 



^ The central tubercle is internally a shallow pit, associated with lucid spots and marking the place of 

 muscular attachment. 



