CONTENTS xi 



Page 



1. Outline of a Natural Classification of the 



Trilobites — Continued 



Family XIII. Cheiruridse 155 



Family XIV. Phacopidae 156 



References 157 



List of Genera 159 



2. The Systematic Position of the Trilobites . . 163 



3. The Larval Stages of Trilobites 166 



Introduction 1^^ 



The Protaspis 167 



Review of Larval Stages of Trilobites 171 



Analysis of Variations in Trilobite Larvfe 179 



Antiquity of the Trilobites 183 



Restoration of the Protaspis 185 



The Crustacean Nauplius 188 



Summary 193 



References 195 



4. On the Mode of Occurrence, and the Structure 



AND Development of Triarthrvs Begki . . 197 



5. Further Observations on the Ventral Struc- 



ture OP Triarthrus 203 



Paired Uniramose Appendages 205 



Anterior Antennae, or Antennules 205 



Paired Biramous Appendages 205 



First Pair of Biramous Appendages, or Posterior 



Antennas 206 



Second Pair of Biramous Appendages, or Mandibles 206 

 Third and Fourth Biramous Appendages, or Maxillae 206 



Thoracic Legs 207 



Organs in the Median Line 208 



The Hypostoma 208 



The Mouth 20D 



The Metastoma 209 



The Anal Opening 209 



Observations 210 



Summary of Ventral Organs of Triarthrus . . . • 211 



6. The Morphology of Triarthrus 213 



References 219 



7. Structure and Appendages of Trinucleus . . . 220 



Appendages 223 



Endopodites 224 



Exopodites 224 



