1869.] GEOLOGY AND MINERALOGY. 343 



resemblances in outline to members of the Limnadiadse, Cypri- 

 dininae, and Cypridae ; in muscle-spot to the first two ; in vascular 

 markings to the first and to the Apodidse ; in the place of the 

 eyes to the second and fourth ; and in the eye-tubercles to the 

 third and fourth. Altogether Leperditia, and its palaeozoic con- 

 geners Isochilina, Eiitomis, Prhnitia, Beyrichia, and Klrkhja, 

 seem to be more nearly within the alliance of the LimnadiadoE. 

 than of the others. Nevertheless, in these, as well as in other 

 groups of Bivalved Entomostraca, we have always to be careful 

 in assigning special value to differences of outline, ornament, and 

 structure, because it is not unusual, among these little Crustacea, 

 to find that similar shells may belong to difi'erent genera, when 

 we examine them alive, and on the other hand very closely allied 

 species may have dissimilar valves. 



As a general rule the fossil Entomostraca of freshwater, 

 brackish, and marine strata, respectively, correspond in family 

 and generic characters to species found in such waters at the 

 present day; and therefore the geologist often finds his sup- 

 position as to the origin of a set of strata confirmed by the 

 presence of this or that kind of Entomostraca ; and in some 

 instances thin intercalated bands of freshwater or of estuarine 

 deposits, amongst marine strata, can be indicated by the presence 

 of EsthericB, which in past, as in present, times appear to have 

 avoided sea-water, though living abundantly in salt-marshes and 

 lagoons. See the ' Monograph of Fossil Estheria,' 1862. 



Thus, also, Mr. G. S. Brady observes (' Intellectual Observer,' 

 1867, p. Ill), iu noticing the geological interest of Entomos- 

 traca, " My belief is, therefore, that those strata which exhibit 

 such very abundant and closely packed remains of the smaller 

 Cypridoi and Cytheridm have most likely been formed in shallow, 

 brackish lagoons, or at the mouths and deltas of rivers. The 

 species of Ostracoda which I have found in these situations are 

 Cytlieridea torosa (Jones), Cy there peUucida, Baird, and Loxocon- 

 cha elliptica, Brady ; while in water a little further from the saline 

 influence, but still slightly partaking of it, it is not uncommon 

 to meet with Cypris salina, Brady, and Cypridopsis aculeata, 

 Lilljeborg, as well as Entomostraca belonging to other orders." 



The Entomostraca act pre-eminently as scavengers in both salt 

 and fresh waters. Most of the groups (as Copepods, Ostracods, 

 and Phyllopods) comprise both marine and fresh-water species ; 

 but the Cladocera are confined to fresh water. The excessive 



