178 



THE CHICAGO ACADEMY OF SCIENCES. 



mentation of the pygidium. The most important structures 

 introduced during these changes in the protaspis itself, are the 

 free cheeks, although in some forms these do not appear until 

 after the close of the protaspis stage. Since the visual area of 



the eyes, when present, is borne 



A & upon the free cheeks, these or- 



gans make their appearance 

 upon the dorsal surface simul- 

 taneously with the free cheeks, 

 and before they appear the free 

 cheeks must be wholly ventral 

 in position. When first dis- 

 cernible the free cheeks are very 

 narrow ; in some forms they in- 

 clude the genal angles, while in 

 others they do not, the genal 

 angles being borne by the fixed 

 cheeks. 



The complete ventral posi- 

 tion of the free cheeks in the 

 earliest larval stages of all but the highest trilobites, indicates 

 this to be a very primitive character, consequently several 

 genera which possess ventral free cheeks as an adult character, 



FIGURE ll. Sao hirsuta Bar- 

 rande. Cambrian; Skrey, Bohemia. 

 A, potaspis. B-F, successive stages 

 in the development to the adult. 

 (After Barrande). 



2 Sao 



Sao 



4 Sao 



1 Sao 



FIGURE 12. Ontogeny of Sao hirsuta Barrande. (Opisthofiarta.} I, 

 Protaspis. 2, Cephalon of a very young individual. 3, Cephalon of an im- 

 mature individual having eight free segments. 4, Cephalon of adult. (From 

 BeecheV, after Barrande.) 



5 Dalmanites 



6 Dalmanites 



Dalmanites 



8 Dalmanites 



FIGURE 13. Ontogeney of Dalmanites seczalis Barrande. (Proparia.} 

 5, Protaspis. 6, Cephalon of individual with three free segments. 7, Cephalon 

 of one with seven free segments. 8, Cephalon of adult. (From Beecher, aftej 

 Barrande.) 



along with other primitive features, may be assumed to occupy 

 a, very low rank, and are grouped together to constitute the 

 most primitive order of trilobites, the Hypoparia. 



