THE CRETACEOUS FORMATION. 
7 
The genera Cyprelld- and Cypridella? have been established by M. de Koninck 
for the reception of certain species found in the Carboniferous strata of Belgium ; 
whilst Entomoconchus^ of Mr, M‘Coy, and BapJmoidea^ of Dr. Hibbert, comprise certain 
branchiopods of the carboniferous system of Great Britain and Ireland. 
General Division — Entomostraca, Muller, 
Legion Branchiopoda/ Latreille. 
Order Ostracoda, Latreille. 1801. Hist. nat. Crust. 
OsTRACiNS on Bitestaces, Vumeril. 1806. Zoologie Analytique. 
OsTRAPODA, Straus. 1821, Mem. du Museum, tom. vii. 
— Milne Edwards. 1830. Annal. Scien., tom. xx. 
OsTRACODES ou Cyproides. — 1834. Hist. nat. Crust. 
Ostracoda, Baird. 1837. Mag. Zool. Bot., vol. i. 
We have already noticed in detail the characteristic parts of the recent ostracodous 
Entomostraca ; we may add that the carapace-valves are more or less oblong in outline, 
longer from the anterior to the posterior extremity than from the superior (dorsal) to 
the inferior (ventral) margin ; they are rarely equilateral, and vary considerably in the 
depth of their internal cavity, sometimes being hemispherical in section, and sometimes 
presenting but an exceedingly shallow cavity. The carapace is always inequivalve, 
the left valve being the largest in nearly every species/ except in those of our 
sub-genus Cytherella, where the opposite condition obtains. The margin of the larger 
valve has a more or less distinct groove or rabbet for the reception of the edge of the 
opposite valve. The mode in which the valves are attached at their dorsal borders varies 
from a simple groove and ridge to more or less complicated hinges provided with 
teeth. In the centre of each of the valves of Cypris, Candona, and Bairdia, in the 
recent state, there frequently exists a patch of lucid spots, varying in size and pattern. 
The nature and use of these spots are at present obscure, the eye never being placed 
opposite or even near to them. Some fossil species of Bairdia exhibit traces of these 
patches, but we have never observed any specimens of Cytherella to afford evidence of 
such spots ; on the contrary, a small tubercle peculiar to Cytherella constantly presents 
1 Descript. Anim. foss, &c., p. 589. ^ Op. cit. p. 590. 
® Journal Geolog. Soc. Dublin, vol. ii, tab. vi. Synop. oharact. &c., p. 164. 
^ Trans. Roy. Soc. Edinburgh, vol. xiii, p. 180. 
“ First applied by Schoeffer to a group of three or four genera of the Entomostraca. 
® Bairdia Siliqua, nob., is an exception, and appears to be the only species, besides the Cytherellse, 
that has the right valve larger than the left. 
7 Mag. Zool. Bot., vol. i, p. 518 ; vol. ii, pi, v, figs. 2, 4, 5, 6, 15, 16*. Lucid spots may be observed 
also in the recent Bairdia subdeltoidea, and, though less distinctly, in Cythere {Bairdia) aurantia, Baird, 
and in the fossil Cythereis interrwpta. 
