CAMBRIAN GEOLOGY AND PALEONTOLOGY 



No. 6.— OLENELLUS AND OTHER GENERA OF THE 

 MESONACID^ 



By CHARLES D. WALCOTT 



(With Twenty-Two Plates) 



CONTENTS 



PAGE 



Introduction 233 



Future work 234 



Acknowledgments 234 



Order Opisthoparia Beecher 235 



Family Mesonacidas Walcott 236 



Observations — Development 236 



Cephalon 236 



Eye 239 



Facial sutures 242 



Anterior glabellar lobe 242 



Hypostoma 243 



Thorax 244 



Nevadia stage 244 



Mesonacis stage 244 



Elliptocephala stage 244 



Holmia stage 244 



Piedeumias stage 245 



Olenellus stage 245 



Peachella 245 



Olenelloides ; 245 



Pygidium 245 



Delimitation of genera 246 



Nevadia 246 



Mesonacis 246 



Elliptocephala 247 



Callavia 247 



Holmia 247 



Wanneria 248 



P.'edeumias 248 



Olenellus 248 



Peachella 248 



Olenelloides 248 



Development of Mesonacidas 249 



Mesonacidas and Paradoxinas 250 



Stratigraphic position of the genera and species 250 



Abrupt appearance of the Mesonacidse 252 



Geographic distribution 252 



Transition from the Mesonacidse to the Paradoxinse 253 



Smithsonian Miscellaneous Collections, Vol. 53, No. 6 



