Smithsonian Report, 1 91 5.— Walcott. 



Plate 14. 



Lower Cambrian Trilobites. 



1. Xevadia ivrcksi (Walcott); 2. 3Icsonacis vermontana{lla\l): 3. Elliptnccphala a.»ijthoid(S Emmons; 4. C'ullavia 

 hrbggeri (Walcott); 6. H(jlinia'kjcridfi(lAnnaxsiiOi\); 6. Wannrria icaknttaniis (Wanner); 7, 8. Pirdciiinkis transitans 

 Walcott. Fig. 8 is an enlargement of the posterior portion of figure 7, showing the rudimentary segments and 

 pygidium beneath the telsonlike segment. 9. Olcnclliis thompsoni Hall. 



Illustrates variation in principal genera of the Mesonacidse; shows changes from most primitive form Kevadia 

 (figure 1) through one line of descent, as represented byfigures 2, 3, and 7, to Olcndlus (figure 9), also another 

 line of descent through figures 1, 3, 4, 5, and 6, probable line of descent from Nevadia (figure 1) to Ilolmia 

 (figure 5), and on to ParadojK7f«. (See pi. 17.) 



