362 GEOLOGICAL SURVEY OF THE TERRITORIES. 



'' Classen 1111(1 Ordnungen derTliierreichs") suggests that tbeLeruiEaris 

 migbt bave infested PalsBOzoic fisb, and on general grounds we sbould 

 think that they probably extended as far back as the primordial 

 zone, inasmuch as highly developed Trilobites and Ostracodes appear 

 there. Another argument is the interesting discovery made in 1865, 

 by Mr. Woodward, of the Cirripede Turrilepas Wrightii from the 

 Wenlock limestone and Dudley shale of the Upper Silurian formation. 

 Previous to this, according to Woodward, "the oldest known Cirripede 

 was the Follicipes rhoetious from the Rhoetic beds of Somersetshire"; 

 while the type is not uncommon in the Cretaceous, and has flourished 

 from that period to the present. 



Of the Merostomata the oldest group is the Eurypterida, the Xipho- 

 sura not dating beyond the Lower Carboniferous. The Eurypterids 

 have not been found below the Upper Silurian (Lower Helderberg in 

 America), and the aberrant forms Hemiaspis, Bunoodes, Pseudoniscus, 

 and Exapinurus are Upper Silurian forms. Among the Xiphosura, 

 Cyclus, the lowest form, is found in the Carboniferous, and ranges, ac- 

 cording to Woodward, as far up as the Permian. In the same period 

 occur Bellinurus, Prestwichia, and Euproops, being in this country 

 found in the lower part of the true Coal-measures, and associated in the 

 same beds with Ceratiocaris, Eurypterus (Anthraconectes and certain 

 Isopoda and Macrurous Decapoda (Anthrapalaemon). The genus Limu- 

 lus first appears in the Jurassic, and the species differ but sbghtly from 

 those now living. 



The more typical Phyllopoda made their appearance during the Triassic 

 period. The lowest group, however, the Estheriadse, appeared during the 

 Devonian, a species referred to Estheria being found in that formation in 

 Europe. The Cladocera are not known to have existed previous to the 

 Tertiary period, and it was not until recently (1862) that Von Hay den 

 discovered the ephippium of a Daphnia in the Eheinish brown coal 

 (Gcrstaecker, in Bronn's Klassen und Ordnungen, &c.), said by Lyell 

 to be of Eocene age. It should be noticed, however, that the fossil be- 

 longs next to Sida, the most highly organized genus of the group, and 

 as it is not unlikely that such pelagic forms as Evadne may have existed 

 in the Mesozoic seas, if not earlier, I have ventured to run the point of 

 the wedge into the Carboniferous period. 



The Apodidse date back to the early part of the Mesozoic, a Triassic 

 species of Apus having been found in Europe, according to Mr. Salter. 



IIL— GEOGEAPHICAL DISTRIBUTION 



(With a map.) 



The materials for the thorough study of the geographical distribution 

 of the Phyllopod Crustacea of Xorth America, as indeed of any other 

 of the continents, perhaps not excepting Buropeo- Asia, are quite scanty. 

 The exceptional habits of the members of this suborder, their usual 

 rarity or periodical occurrence, and their very local distribution, have 

 caused them to escape the observation of most collectors, and to be found 

 more by accident than as the result of well-matured plans of search. 



The salient points in the distribution over the globe of the Phyllopods 

 are as follows; although the conclusions here presented are, of course, 

 provisional, and much yet remains to be discovered as to the distribu- 

 tion of these interesting forms. 



It will be seen by reference to the lists presented in the following 

 pages that a large proportion of our Xorth America Phyllopoda, includ- 

 ing nearly all the species of Ustheria, are restricted to the elevated dry 

 central zoogeographical province of the United States, and adjacent 



