DISTRIBUTION OF CRUSTACEA. 273 



present day — Estheria, Cythere, Bairdia, Cypridina — thus present- 

 ing one of the most remarkable instances of the persistence of 

 type-structure known in the whole range of the animal kingdom. 

 The genus Estheria dates from the Devonian period, and attains 

 its maximum development in the Trias. In its modern distribu- 

 tion it may be said to be almost cosmopolitan, although it would 

 seem to prefer the regions of warm climate, and not to penetrate 

 much beyond the fifty-fifth parallel of latitude. The range of some 

 of the species is extraordinary. Estheria Dahalacensis, which oc- 

 curs as far north as Vienna, is found from Sicily to the island of 

 Dhalak, in the Red Sea, or over an area whose extent is meas- 

 ured by about thirty degrees of longitude, and thirty-two degrees 

 of latitude. The range of E. tetracera comprises fully forty de- 

 grees of longitude (Oran — Kharkov), but is more than equalled 

 by the Carboniferous species E. Leidyi, which has been reported 

 from both England and the State of Pennsylvania. It is a singular 

 circumstance that while all the recent species of Estheria are in- 

 habitants of fresh water, or of water which is but barely brackish, 

 the fossil forms are frequently, or generally, found associated with 

 distinctively marine types of organisms, indicating apparently for 

 these species also a marine habit. While this may have been 

 true, the association with fresh-water forms, which also occurs, 

 tends to show that it was only partially the case. 



Possibly belonging to the order of the Phyllopoda, but by 

 some authors placed among the Malacosti-aca, are the singular 

 shield - bearing crustaceans of the Silurian period, Ceratiocaris, 

 Dictyocaris, Discinocaris, Peltocaris, &g., whose afiinities have been 

 placed with the modern genera Nebalia (marine) and Apus (fresh- 

 water). In Discinocaris the shield in some individuals measures as 

 much as six inches across. Hymenocaris, which is exclusively 

 Cambrian, represents the oldest type of this order of crustaceans. 

 The genus Apus itself, which is almost universally distributed, and 

 has been observed even in Norway at an elevation exceeding throe 

 thousand feet, appears as early as the Carboniferous period. Bran- 

 chipus, although devoid of a head-shield, is stated by Woodward 

 to be preserved as a fossil in the Eocene (Oligocene ?) beds of the 

 Isle of Wight."' 



Of the more imjiortant genera of recent Ostracoda, Cythere and 

 Bairdia both date from the Silurian period, and Cypridina from 

 13 



