No. 634] PHYLOGEXY OF THE ARTHROPOD A 411 



remarkable assemblage found by Walcott at a single lo- 

 cality in British Columbia. In this fauna the Crustacea 

 are represented by trilobites, anostracan and notostracan 

 branchiopods, Marrella, Limulava (possible ancestors of 

 the merostomes), Aglaspidae (possible ancestors of the 

 Xiphosura), and Leptostraca, the most primitive Mala- 

 costraca. The Upper Cambrian brought the first true 

 Merostomata. In the Ordovician, Ostracoda and Cirri- 

 pedia first appear, and in the Silurian the first undoubted 

 Xiphosura, primitive Diplopoda, and Scorpiones. In- 

 secta and air-breathing Arachnoidea, including Araneae, 

 appear suddenly in the Pennsylvanian (Upper Carbonif- 

 erous), and the oldest known Chilopoda are found with 

 them. All of the tracheates probably have a long pre- 

 Pennsylvanian history, however, and the record of the 

 fossils is liable to be amplified by new discoveries at any 

 time. 



The geological record, so far as it is now available, is 

 in favor of the theory that the other Arthropoda were 

 derived from the trilobites, for although Crustacea were 

 highly diversified by the Middle Cambrian, all other than 

 these were rare, and the trilobites, while they had not 

 reached their highest development, were exceedingly 

 abundant and varied. 



If the trilobites were the most primitive arthropods, 

 the question of the ancestry of the phylum resolves itself 

 into a search for the progenitors of the former. What 

 would be the form of the animal from which the trilobite 

 was derived? The depressed form universal in the sub- 

 class and the equally universal lateral (' 'pleural") lobes 

 have already been commented upon. From a study of 

 comparative morphology, it appears that the more an- 

 cient trilobites, and the more ancient members of higher 

 families within the subclass, have the most nearly flat 

 form, the narrowest axial, and the widest pleural lobes. 

 Turning to ontogeny, it is found that in most cases the 

 protaspis of any species shows the same characteristics. 

 All these suggest a broad depressed animal with narrow 



