176 STUDIES IN EVOLUTION 



outline and axis similar to Acidaspis. The eyes are quite 

 large and situated, as in the same stage of Proetus, transverse 

 to the axis, on the anterior border. Genal angles present, but 

 in this case not produced by the free-cheeks as in Sao and 

 PtycJioparia ; glabella strongly annuhited, increasing in diam- 

 eter anteriorly, although not expanding at the frontal mar- 

 gin as in Sao^ etc. In the two following stages (figures 9, 10), 

 the pygidium increases in size, and the pleura are defined. 

 To reach maturity (figure 11), eleven segments are devel- 

 oped in the thorax, the glabella becomes more prominently 

 developed in front, but the five annulations are maintained. 

 The eyes have travelled in and back as far as the third cepha- 

 lic segment, and their longer axes have swung around into a 

 position parallel with the axial line, as in Proetus. The py- 

 gidium has added many new segments, and the extremity is 

 prolonged into a spine. 



Before proceeding further in the discussion of the protaspis, 

 it is necessary to notice a number of forms of young trilobites 

 which have heretofore been referred to the embryonic and 

 larval stages, but which are now believed to belong to stages 

 later than the protaspis. 



Besides the truly elementary forms described by Barrande 

 and already noticed (^Sao Mrsuta and Dalmanites socialis), 

 there are others which he referred to his second, third, and 

 fourth orders of development. ^ Among these Agnostus may 

 be taken first. The youngest forms of Agnostus nudus and 

 A. rex (figures 76, 77) measure respectively 2 and 1.3 mm. 

 in length, and the adults 13 and 15 mm. The earliest stages 

 of the genera shown on Plates III and IV measure less than 

 1 mm., while the adults are more than 25 mm., with the 

 exception of Proetus parviusculus, wliich is seldom more than 

 10 mm. long, though this species has a protaspis .72 mm. in 

 length. The cephalon and pygidium of the youngest known 

 Agnostus are quite separate and distinct, v/hich is not the 

 case with the typical protaspis stage. It therefore seems 

 probable that on account of the comparatively large size and 



