CAMBRIAN GEOLOGY AND PALEONTOLOGY 



II 



No. 6.— MIDDLE CAMBRIAN BRANCHIOPODA, 



MALACOSTRACA, TRILOBITA, AND 



MEROSTOMATA 



By CHARLES D. WALCOTT 

 (With Plates 24 to 34) 



CONTENTS PAGE 



Introduction 148 



Habitat 149 



Character of the shale 149 



Mode of occurrence 151 



Classification 153 



Stratigraphic distribution 155 



Structural features 157 



Exoskeleton 157 



Labrum 1 58 



Segmentation 1 58 



Appendages 1 58 



Alimentary canal 160 



Hepatic caeca 160 



Origin of Middle Cambrian crustacean fauna 160 



Relation to recent crustaceans 164 



Survival of the Branchiopoda 165 



Class Crustacea 166 



Sub-Class Branchiopoda j66 



Order Anostraca Caiman 166 



Family Opabinidse, new family 166 



Genus Opabinia, new genus 166 



Opabinia regalis, new species (pis. 27, 28.) 167 



Appendages 167 



Interior structure 168 



Dimensions 168 



Female 169 



Opabinia ? media, new species 1 70 



Genus Leanchoilia, new genus 170 



Smithsonian Miscellaneous Collections, Vol. 57 No. 6 



145 



i 



