TYPE IN WHICH THE ADULT PYGIDIUM DEVELOPS LATE. 



335 



glabella is not yet distinct. At its sides, close to the anterior 

 margin of the body, two arched indentations can be recognised ; 

 these, it may be conjectured, are connected with the rudiments of 

 the eyes (a). The rudiment of the cephalic shield forms the chief 

 part of the body at this stage. The posterior section is small and 

 shows the rudiments of a few indistinct segments. A few tooth-like 

 projections of the posterior margin are to be regarded as the pleura 

 of these segments. This region contains the rudiment of the whole 

 of the future thorax and pygidium. 



Jl. 



^ 



B 



%^2 



Fig. 150. — Five ontogenetic stages of .Sao Idrsuta (after Barrande). A, youngest stage. B, 

 somewhat older stage with more distinct demarcation of the cephalic region. C, stage with 

 two free thoracic segments. D, stage with seven free thoracic segments. E, stage with 

 twelve free thoracic segments, a, rudiment of eye ; g, facial suture ; p, transitory pygidium. 



In a later and somewhat larger stage (Z?) the cephalic region has 

 become marked off by a distinct boundary from the posterior region. 

 In this latter, the rudiments of the segtnents have become more 

 distinct, and thev have increased in number. In the succeedino- 

 stages (C, D, E) the thorax first makes its appearance as a distinct 

 body-region, being formed from the anterior segment-rudiments of 



