“f ' Monthly Mi ical 
184 On Ancient Water-fleas of the SOUL Oct ted. 
II.—On Ancient Water-fleas of the Ostracodous and Phyllopodous 
Tribes (Bivalved Entomostraca). 
By Professor T. Ruprrt Jonzs, F.GS. 
Pirate LXI. 
Part I.—The Leperpitiap2. 
Tue “ water-fleas”” are a mixed group of minute Crustaceans, some 
in fresh and others in salt water, which received their common 
English name from their jerky movements, and their continental 
name of “ Entomostraca,” or “shell-insects,” from their somewhat 
insect-like appearance and their shell-like cases. «The “water-fleas” 
proper, however, are the Daphniadz and their near allies, mostly 
of fresh-water habits. These have for the most part a very delicate, 
thin, horny envelope, or carapace, bent down on either side oyer, 
and enclosing, the body, but not divided and jointed along the back. 
Of these there are no well-known fossil representatives ; but certain 
remains of such soft-shelled water-fleas are recorded by M. Mahony, 
in the ‘ Geological Magazine, No. 63, p. 392, as occurring in a 
Post-tertiary clay in Renfrewshire; and some possibly Daphnioid 
Entomostracans are noticed in the same Magazine, No. 71, p. 219, 
as coating a layer of coal-shale from South Wales. Some old fossils 
have been referred to Daphnia on a mistaken notion of similarity 
of form. 
Other water-fleas are termed “ water-shell-fleas”* (Cypris, &c.) 
and “sea-shell-fleas” (Cythere, Cypridina, &c.), hayimg more dis- 
tinct shells, with two valves, jointed on the back-line. These are 
abundant enough fossil in many strata of different ages. 
The English names, however, are too general for special use in 
indicating the different kinds, the family and generic distinctions 
being very numerous and decided in this important sub-class of the 
great Crustacean group or class. Indeed it comprises these and 
other many-featured, minute, aquatic, articulate creatures, for which 
the term Hntomostraca has been accepted as the systematic name. 
Of these the really bivalved forms (namely, the Ostracoda and 
some of the Phyllopoda) have left their carapace-valves in the muds 
of lakes and seas of all ages as far as the history of the globe can be 
traced back geologically. The geologist recognizes three main 
stages of the world’s progress, which he terms the Paleozoic (old 
life) or Primary, the Mesozoic (middle life) or Secondary, and the 
Cenozoic (new life) or Tertiary and Post-tertiary. These periods 
had their own characters of life, as shown by the fossils preserved 
in the respective groups of successive strata; and as regards the 
Bivalved Entomostraca there are many noticeable facts as to the per- 
* Catalogue of Crustacea,’ in the British Museum, 12mo, 1850. 
