8UB-CLAS- I TIM Ll U'dTA 819 



as indicated by the additional ,- ami lirnl*. In the earliest. 



or Cambrian genera, the protaspia tage i- i>v far the simplest e\pr in of 



this period to 1)0 found. In the higher ami lat. c8 ef 



acceleration or earlier inheritance has pu.-hed forward rrrtain character- until 

 they appear in the protaspis, thus making ii more ami moiv complex. 



Taking the early protaspi.- Btagefl in >'"/>/,///, ,//,/. /,/'os//v/f*/s, or /'///<7/<////-/'</. 

 it is found that they agree exactly with the foregoing dia^no.-i- in it.- mo.-t 

 elementary sense. Since they are the character! shared in common l>v all 

 larvae at this stage, they are taken aa j)rimitive, and accoi-ded that value in 

 dealing with adult forms possessing homologous features. Therefore, any 

 Trilobite with a large elongate cephalon, eyes rudimentary or absent 

 cheeks ventral or marginal, and glabella 

 long, cylindrical, and with five annulation>, 

 would naturally be placed near the beginning ^ 



of any genetic series, or as belonging to a ^2j| (^ 



very primitive stock. 



Next must be considered the progressive * 

 addition of characters during the geological 

 history of the protaspis, and the ontogeny 

 of the individual during its growth from 

 the larval to its mature condition. It has 

 been shown by Beecher that there is an 

 exact correlation to be made between the 

 geological and zoological succession of first 

 larval stages and adult forms, and therefore 

 both may be reviewed together. Sao Mnuta, Ban. caiiii.nan ; skiv\. 



rrn n ' Bohemia. A, Protasiiis. /.'-/'. NVnionic stages 



The first important structures not fderelopraelit(aftrBnnde). 



especially noticeable in all stages' of the 



protaspis are the free cheeks, which usually manifest themselves in the meta- 

 or para-pro taspis stages, though sometimes even later. Since they bear tin- 

 visual areas of the eyes, when such are present, their appearance on the dorsal 

 shield is practically simultaneous with these organs, and before the eyes have 

 travelled over the margin, the free cheeks must be wholly ventral in position. 

 When first discernible, they are very narrow, and in Ptychoparia and Sao, 

 include the genal angles. In Dalmaniles and Cheimrus, however, the genal 

 angles are borne on the fixed cheeks. 



Since the free cheeks are ventral in the earliest larval stages of all but the 

 highest Trilobites, and as this is an adult feature among a number of genera, 

 which on other grounds are very primitive, this is taken as generally indicative 

 of a very low rank. The geneir Y/"/yw.<, ?lifiti>t>fHs t Trinucleus, and their allies 

 agree in this respect, and constitute the Hypoparia. 



The remaining genera of Trilobites present two distinct types of head 

 structure, dependent upon the extent and character of the free cheeks. In 

 both, the free cheeks make up an essential part of the dorsal crust of the 

 cephalon, being continued on the ventral side only as a doublure or infolding 

 of the edge, similar to that of the free edge of the cranidium, the ends of the 

 thoracic pleura, and the margin of the pygidium. They may be separate* 1 

 only by the cranidium, as in Ptychoparia; by the cranidium and separate 

 epistomal or rostral plate, as in Illaenus and Homalonotus ; or they may In- 

 united and continuous in front, as in Aecjlina and Ihiluwnit''*. One type of 



