186 On Ancient Water-fleas of the — [Monthly Microscopical 
characters not present together in later forms, but to be found in 
separate families and genera. 
The Paleozoic strata (grouped as Laurentian, Cambrian, 
Silurian, Devonian, Carboniferous, and Permian) have yielded many 
genera of Bivalved Entomostraca, most of which have been described 
and figured; but some remain unpublished. Among these old bi- 
valved carapaces and separate valves there are many that present 
such shapes and features as now belong to Cythere, Cypris, Cypri- 
dina, and their congeners ; and it is very difficult, if not impossible, to 
form definite conclusions on imperfect evidence as to the exact 
grouping that should be adopted, in referring the fossil valves either 
to recent or to extinct genera ; for, as has often been noticed, the 
general aspect of such carapaces is not only an ambiguous, but even 
a false guide; the minute distinctions of hingement, overlap, muscle- 
spot, and ornament being obscure, and the important characters of 
limbs and infernal organs wholly wanting. 
The following Table gives an outline of the classification of the 
Entomostraca : *— 
CLASS CRUSTACEA. 
[Norz.—Those marked with an asterisk are known only in the fossil state. ] 
Sus-Ciass 1. Decapopa (Cancer, &c.). 
2. TeETRADECAPODA (Oniscus, &c., Trilobita ?*). 
3. ENTOMOSTRACA. ‘ 
Orper I. Gnathostomata. 
Sus-OrvER 1. Lophyropoda. 
Tree I. Cyclopoidea (Copepoda). 
Families. Cyclopidx, &c. 
Tripe I. Daphnoidea (Cladocera), 
Families. Penilide, Daphniade, Bosminidx, Lynceidx, &c. 
Trize III. Cyproidea (Ostracoda). [Bivalved.] 
Family 1. Cypride. 
Genus. Cypris. Genus. Notodromas. 
Chlamydotheca. Candona. 
Cypridopsis. Pontocypris. 
Potamocypris. Argilloecia. 
Paracy pris. Bairdia. 
Aglaia. Macrocypris. 
* The recent works of Mr. G.§. Brady should be consulted for the newest 
information on the living Ostracoda. 
