6 THE ENTOMOSTRACA OF 



Ireland by Mr. M'Coy,^ and two species from the Maestricht limestone by 

 M. Bosquet.^ 



Of the fossil Entomostraca nothing remains but the carapace-valves ; it is, there- 

 fore, by these alone that specific and even generic distinctions can be recognised. 

 And although the general form and markings, used by Dr. Reuss in his classification, 

 cannot alone be depended upon in subdividing this group, yet the peculiar characters 

 of the dorsal margins of the valves are sufiiciently distinct and constant to enable us 

 to arrange these animalculites into sections which bear considerable, if not perfect, 

 analogy to the subdivisions of the recent genera. We have therefore grouped the 

 Cretaceous species into four sections, according to the characters of the hinge. 

 These sections, although well marked, can scarcely rank as true genera, on account of 

 the absence of all knowledge of the soft parts, on which the division of the recent 

 branchiopods into genera depends. 



Ostracoda having carapaces characteristically distinct are at present united in some 

 of the recent genera ; and besides the peculiar forms of carapace of existing species 

 there is at least one other also present among the fossil Ostracoda. These peculiarities 

 in the structure of the carapace- valves are especially apparent at the contact-margins 

 and the dorsal borders ; the former varying very much in their mode of adaptation to 

 one another, and the latter being either simple or provided with a more or less com- 

 plicated hinge. These characters have not hitherto been regarded as generic dis- 

 tinctions among the recent Ostracoda ; but in the fossil species of this order, where the 

 carapace-valves are our only guide in arrangement, these structural differences are of 

 primary importance. 



Three forms of carapace in particular are present among the recent Cytheres and 

 exist also among their fossil congeners. We have reserved the term Cythere for the 

 form belonging to four of the five species of Cythere figured and described by Miiller, 

 the first two and the last two, viz., Cythere viridii and C. luteal C. gibhd' and 

 C. gihhera^ Miiller ; also to C. reniformisj Baird, &c. The form peculiar to Cythere 

 variabilis^ and C. aurantia,^ Baird, and perhaps C.Jlavida^^ Miiller, belongs to the 

 group composing Mr. M'Coy's genus Bairdia ^^ but in accordance with the views 

 above mentioned we can at present regard it as a sub-genus only. Cythereis is a 

 sub-genus, comprising a third form, rare among the recent Ostracoda (and hitherto 

 unpublished), but plentiful in the Cretaceous and Tertiary formations. Lastly, another 

 distinct form, found fossil only, constitutes our sub-genus Cytherella. 



^ Under the name oi Daphnia prim(eva, Syn. charact. &c. p. 164. 



2 Described as CyprellcB by M. Bosquet, op. cit. p. 22. 



^ Entomostr. tab. vii, figs. 1, 2. * Op. cit. tab.vii, figs. 3, 4. 



5 Op. cit. tab. vii, figs. 7-9. « Op. cit. tab. vii, figs. 10-12. 



7 Mag. Zool. Bot., vol. ii, pi. v, fig. 17. « Op. cit. pi. v, fig, 25. 



^ Op. cit. pi. V, fig. 26. 10 Entomostr. tab. vii, figs. 5, 6. 



11 Synop. charact. &c., p. 164, 1844. 



