50 Rupert Jones, on Bivalved Entomostraca. 



also. The persistence of these genera from so old a time to 

 the present is what is expected of such relatively low forms 

 of life ; wide geographical extension and long-continuance 

 belonging to such creatures as have not been highly spe- 

 cialised. In the " Permian " formations (" Magnesian Lime- 

 stone " of Durliam and other strata; Buirdia, Cythere, and 

 Kirkhya play an important part. In the " Trias " or " New 

 Red Sandstone " we find Estheria, where marine conditions 

 failed and fresh Avater had an influence, not only in Europe, 

 but in India and America. (See my ' Monograph on Fossil 

 Estherite,' 1862.) The Entomostraca of the " Lias " and the 

 " Oolites " are not few, though not well known. In the 

 "Pm-beck" and " Wealden " beds they are better known. 

 Masses of Purbeck building stone are wholly composed of 

 the valves, and some of the Weald clays split like paper along 

 the layers of shed valves of Cypridea ; nor are Estherice want- 

 ing in these old fresliAvater beds. The " Gault " and 

 " Chalk " are full of Cythere, Bairdia, and other allied 

 genera, all marine. The " London Clay," the " Brackles- 

 ham Beds," and " Barton Clay," swarm in some places with 

 similar forms, whilst the " Woolwich Beds " below them, 

 and the " Hampstead " and " Osborne " formations of the 

 Isle of Wight, above, are characterised by Can dona, Cythe- 

 ridea, &c., such as love estuaries, lakes, and rivers. Lastly, 

 for England, the " Crag " of Suffolk, and that of Bridlington, 

 abound in marine forms. 



If we had only these little fossils whereby to form an 

 opinion of the probable conditions under which the clays, 

 sandstones, and limestones were formed in the long past eras 

 of this planet, we should have, in nearly every case, ample 

 evidence of the history of each bed of mud, silt, and shell- 

 sand, in which these minute Entomostraca can be found. 



The seas of the Silurian period had their thick-shelled 

 Leperditi(B and Beyrichia very distinct from* their noAv living 

 congeners, but linked to them by close affinities readily dis- 

 coverable by the naturalist. When land was increased, in 

 the Devonian period, the sea-coasts still abounded with marine 

 Crustacea; and the lakes and rivers abounded with -EsMeri^e, 

 like those of the present day. The coral-seas, which gave 

 birth to the Derbyshire limestone, abounded with strange 

 forms of Entomostraca. Land still extended, and miles and 

 miles of swampy coasts and lowlands crowded with the 

 dense vegetation of the Coal-period, and, intersected A^dth 

 black, muddy lagoons, offered a home for endless tribes of 

 Entomostraca, feeding on animal and vegetable refuse — the 

 rotting plants and shoals of fish, poisoned by the black mud 



