THE CRETACEOUS FORMATION. 7 



The genera Cyprellc^ and CypridelloP- have been estabhshed by M. de Koninck 

 for the reception of certain species found in the Carboniferous strata of Belgium ; 

 whilst Eutonioconchus^ of Mr. M'Coy, and Baphioidea'^ of Dr. Hibbert, comprise certain 

 branchiopods of the carboniferous system of Great Britain and Ireland. 



Getieral Division — Entomostraca, Milller. 



Legion Branchiopoda,^ Latreille. 



Order Ostracoda, Latreille. 1801. Hist. nat. Crust. 



OsTEACiNS ou BiTESTACES, Dumeril. 1806. Zoologie Analytique. 

 OsTRAPODA, Straus. 1821. Mem. du Museum, torn. 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 Cytlierella, 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 Cytlierella 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. cbaract. &c., p. 164. 



■* Trans. Roy. Soc. Edinburgh, vol. xiii, p. 180. 



' First applied by Seboeffer to a group of three or four genera of the Entomostraca. 



8 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. 



^ 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 suhdeltoidea, and, though less distinctly, in Cythere (Bairdia) aurantia, Baird, 

 and in the fossil Cythereis interrwpta. 



