SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 53 



Mesonacidae — Continued. page 



pygidium of 245-246 



stratigrapliic position of the genera and species 250, 251 



thorax of discussed, defining various stages of development 244-245 



transition to Paradoxinse 253 



visual organs on hypostoma of 241-242 



Mcsonacis Walcott 261 



Cole, in synonymy 261 



Moberg, in synonymy 261 



Peach and Home, in synonymy 261, 267 



Walcott, in synonymy 261 



Weller, in synonymy 262 



compared with ElHptoccphaht 269 



Nevadia 257 



Olencllus 304 



Pccdcumias 266, 304, 306 



Wanncria 298 



delimitation of genus 246 



development of, shown in diagram 249 



development of thorax in 244 



line extinct in Lower Cambrian 249 



mentioned 233, 236, 245, 247, 248, 250, 263, 288, 295, 306 



species referred to the genus listed 232 



stage in development of thorax defined 244 



stratigraphic distribution tabulated 251 



mickivitzi ( Schmidt) 262, pi. 26, fig. 4 ; text figs. 16 and 17 



Peach, in synonymy 262 



compared with Mcsonacis vcniionfaiia 263 



generic relations of 263 



mentioned 247 



stratigraphic distribution tabulated 251 



torelli (Moberg) 264, pi. 26, figs. 5-18 



compared with Olencllus F, sp 342 



hypostoma of 244 



mentioned 247, 341 



stratigraphic distribution tabulated 251 



vermontana (Hall) 264, pi. 26; figs. 1-3 ; pi. 44, fig. 2 



Marr on posterior segments of 313 



Moberg, in synonymy 266 



Walcott, in synonymy 265 



compared with Mcsonacis mickzi'itci 263 



Nevadia tvccksi 260 



Olencllus ? gigas 324 



Olencllus thonipsoni 338 



Pccdcumias transitans 306, 308, 338 



geographic distribution of 253 



mentioned 247, 250, 256, 264, 269 



posterior portion of compared with tclson of Olencllus fhoinf- 



soni 233, 26(3 



stratigraphic distribution tabulated 251 



