172 STUDIES IN EVOLUTION 



from the Cambrian of Nevada and Utah. Figure 5 represents 

 a cast of the protaspis, and shows a defined occipital ring, 

 with the axis slightly expanded and undefined in front ; py- 

 gidium truncate behind. Figure 6, which is referred to a 

 later stage (metaprotaspis) of the same species, shows the 

 inception of several characters that have not as yet appeared 

 in the previous larvae. The axis is very strongly annulated ; 

 the anterior lobe is nearly as long as the four posterior 

 annulations of the cephalon, and on each side there is a 

 furrow representing the eye-line of the adult ; the free-cheeks 

 are present as narrow marginal plates, including the genal 

 spines; the pygidium shows two segments separated by a 

 furrow. 



An adult Ptychoparia Kingi is shown in figure 7, and may 

 be taken as representing the sum of the changes passed 

 through in the development of larvae like the preceding, 

 belonging to the genera Solenopleura^ Liostracus, and Ptycho- 

 paria. The introduction and growth of the segments of the 

 thorax are perhaps the most marked changes, but other points 

 of importance to be noted are: the comparatively smaller 

 size of the cephalon and its transverse form ; the limitation 

 and recession of the glabella, which is now rounded in 

 front, and only extends about two-thirds the length of the 

 cephalon; the growth of the eyes and free-cheeks at the 

 expense of the fixed-cheeks ; the increased segmentation of 

 the abdomen, shown in the axial and pleural grooves on the 

 pygidium. 



Sao hirsuta Barrande ; Plate ITT, figures 8, 9, 10, and 11 ; 

 from the Cambrian of Bohemia ; after Barrande. 3 The speci- 

 mens of this species are preserved as casts, and several of the 

 features are therefore somewhat subdued. The earliest or 

 anaprotaspis stage, represented in figure 8, is quite as primi- 

 tive in most respects as any of the preceding. It is circular 

 in outline, the annulations of the axis are distinctly shown 

 only in the neck segment and pygidial portion, and the eye- 

 line is present. In figure 9 of the metaprotaspis quite an 

 advance is seen in the development of the free-cheeks and 



