CONTEXTS. 



Page. 



Preface ix 



Introduction 1 



Fossil medusas of the Middle Cambrian terrane 3 



Mode of occurrence 3 



Condition and manner of preservation 4 



Relation to otber fossil medusae 8 



Relation to living medusa' 9 



Litbologic characters of the siliceous nodules 11 



Mode of occurrence of the siliceous nodules . . 13 



Chemical character of the nodules and shale 14 



Silicificatiou 15 



Source of the silica 17 



Relation to the sponges 21 



Description of genera and species 22 



Genus Brooksella 22 



Genus Laotira 31 



Fossil medusa? of the Lower Cambrian terrane of eastern New York 41 



Genus Daetyloidites 41 



Fossil medusae of the Lower Cambrian terraues of Sweden and Bohemia 47 



Description of genera and species 49 



Genus Medusina 49 



Genus Eophyton 59 



Fossil medusa' from the Jurassic and the Permian 65 



The .1 urassic 05 



Origin of the impressions of medusa' 68 



Classification 69 



Description of genera and species 70 



Genus Semoeostomites 70 



Genus Enlithota 73 



Genus Acraspedites 75 



Geuns Rbizostomites 7(5 



Genus llexarhizites 86 



Genus Leptobrachites 88 



Genus Medusina 91 



The Permian 95 



Genus Medusina 95 



Incerta' sedis 96 



Cretaceous 97 



Genus Mednsites 97 



Genus Medusichnites 100 



Genus 1 liscopby Hum 101 



v 



30662 



