410 THE AMERICAN NATURALIST [Vol. LIV 



presence of two pairs of antennae does not point directly 

 to the trilobites, but to some offshoot like Marrella, and it 

 is possible that the line has been : trilobite, Marrella-\ike 

 marine animal, chilopod-like tracheate, insect. 



There remain only the Diplopoda, which show a few 

 trilobite-like characteristics, notably their lateral out- 

 growths and the endopodite-like walking legs on every 

 segment. Antennules are present, antennae absent, man- 

 dibles and maxillulse much modified, the latter possibly 

 biramous, and maxillae absent. The most characteristic 

 feature, the possession of two pairs of limbs on each seg- 

 ment on a part of the trunk, can be shown to have arisen 

 comparatively recently (geologically), for Silurian and 

 Devonian fossils which are undoubted diplopods have a 

 test like that of a trilobite and eyes much like those of a 

 Phacops. While there are no close connections, there is 

 nothing to show that the Diplopoda could not have been 

 derived from the Trilobita. 



Summary 



After the above survey, it should appear that the trilo- 

 bites, particularly in respect to their appendages, are 

 more primitive than any other Arthropoda. The chief 

 modifications in other groups are in the nature of reduc- 

 tions, in the loss of whole appendages, of branches or of 

 segments. Extra segments are sometimes added in cer- 

 tain appendages, and new outgrowths, epipodites, are 

 common among the Crustacea. The trilobites have what 

 seems, at first sight, a peculiar and specialized dorsal 

 test, but now that it has been shown that the pleural lobes 

 may be lost and the pygidium reduced to a single seg- 

 ment, and, chiefly, that the wormlike form is not primi- 

 tive but secondary, they may be viewed in an entirely 

 new light. 



In the oldest fossiliferous rocks, Lower Cambrian, 

 trilobites are plentiful, branchiopods rare, and no other 

 arthropods present. A greater differentiation is seen in 

 the Middle Cambrian fauna, due, however, entirely to the 



