the Paleozoic Bivalved Entomostraca. 61 
on the figure is an exaggerated feature, and without import- 
ance; in other respects the specimen somewhat resembles our 
new Cythere Jukestana, but it is much shorter in proportion 
and more arched. It also approaches some of the Bairdie in 
shape; but its narrow (anterior) extremity has the curvature of 
a Cythere, and is markedly deficient in the peculiar up-turned 
hatchet-like edge characteristic of Batrdia. 
C. Aldensis is smaller (;!; inch long) and less convex than 
the specimen of Primitia Maccoyzi associated with it in the 
same limestone. 
The name “ Cytheropsis” has been applied to this and 
other Paleozoic Entomostraca*. With regard to this term as 
a generic appellation, we once thought it useful in classifying 
those Paleozoic Entomostraca that do not closely assimilate 
either to Leperditia or Beyrichia, but in outline and size re- 
semble many of the Cythere of existing seas, though differing 
- from them in having tubercles, relatively thick valves, or other 
distinctive features (Ann. Nat. Hist. ser. 3. vol. i. p. 249). The 
establishment, however, of the natural group of Primitie en- 
ables us to brig together several of the “simple Beyrichie,” 
some of the dubious Leperditioid forms, and nearly all the so- 
called Cytheropses. Indeed of the known species referred to 
Cytheropsis there remain only C. rugosa (Jones, Ann. Nat. 
Hist. ser. 3. vol.1. p. 249, pl. 10. fig. 5), which is probably a 
Primitia, figured upside down, and C. siliqua (Jones, op. cit. 
fig. 6), which, perhaps, like some of the Kildare specimens, is 
a Cythere or a Macrocypris. Kxcepting the relatively greater 
thickness of the valves in some of them (and that is more ap- 
parent than real), there is nothing to indicate that these old 
Entomostraca, which “ Cytheropsis” was intended to com- 
prise, differed from what now exist as Cythere, Bairdic, 
Macrocyprides, &c. The so-called Cytheropses of the Car- 
boniferous formations have already been shown to belong to 
Leperditia Okeni, &c. (see Ann. Nat. Hist. ser. 3. vol. xviii. 
p- 39). 
EXPLANATION OF PLATE VIL. 
Fig. 1. Primitia Maccoyii (full-grown): a, right valve; 6, dorsal, and 
ec, ventral aspect. 
Fig. 2. P. Maccoyii (intermediate stage of growth): a, left valve; 6, ven- 
tral aspect. 
Fig. 3. P. Maccoyit (young): a, left valve; 6, ventral aspect; ec, end 
view. 
*“Cytheropsis” has also been applied to a group of recent Cytheride by 
G. O. Sars in 1865; but G.S. Brady proposes Lucythere in its place for 
these living forms. 
