THE TERTIARY FORMATION. 



Division — Entomostraca, Milller} 



Animal aquatic, covered with a shell or carapace of a horny consistence, formed of 

 one or moi^e pieces ; in some genera resembling a cuirass or buckler, and in others 

 a bivalve shell, which completely or in great part envelopes the body and limbs of the 

 animal ; in other genera the animal is invested with a niultivalve carapace, like jointed 

 plate-armour : the branchiae are attached either to the feet or to the organs of masti- 

 cation ; the limbs are jointed, and more or less setiferous. The animals, for the most 

 part, undergo a regular moulting or change of shell as they grow ; in some cases this 

 amounts to a species of transformation.^ 



Tlie following is Mr. Dana's classification of this division : 



Division — Entomostraca. 



Order 2. Cormostomata. 

 Legion I. Poecilopoda. 

 ,, II. Arachnopoda. 



Order 1. Gnathostomata. 

 Legion I. Lophyropoda. 

 Tribe 1. Cyclopoidea. 

 „ 2. Daphnioidea. 

 „ 3. Cyproidea. 

 Legion II. Phyllopoda. 



I have elsewhere observed that, in the case of the fossil Entomostraca, the soft 

 parts, including the branchial, maxillar}^, and locomotive organs, on which the generic, 

 and sometimes the specific, distinctions of the recent forms are mainly established, 

 have quite disappeared, the hard carapace-valves alone remaining to guide us in the 

 recognition of genera and species. It is fortunate, however, that the families, and 

 most of the genera even, of the existing bivalved Entomostracans have carapaces 

 sufficiently characteristic to enable us to co-ordinate the fossil forms by the analoo-ies 

 presented in the form and structure of the valves. When we refer, however, to the 

 minute distinctions of form, hingement, and ornamentation, we find that among the recent 

 bivalved Entomostraca some families, and even genera, have carapaces peculiar to 

 them {Nelialia, Linmadia, and Cypridina) ; whilst in other families a nearly similar 

 carapace belongs to two genera {Cypris and Candona, DapJmia and Lynceus) ; and, on 

 the contrary, even two characteristically different carapaces occur among the species 

 of one genus [Cythere and its sub-genus Bairdia). 



In the case of two or more genera presenting a similar form of carapace, we 



' For synonyms see Dr. Baird's 'Nat. Hist. Brit. Entom.,' p. 16. 



2 Zenker observes that tliere is no casting of the carapace in the Ostracoda. 



