LARVAL STAGES OF TRILOBITES 181 



marked development of muscular fulcra, which are supposed 

 to be connected with the hypostoma. 



The next structures not especially noticeable in all stages 

 of the protaspis are the free-cheeks, which usually manifest 

 themselves in the meta- or paraprotaspis stages, though some- 

 times even later. Since they bear the visual areas of the 

 eyes, their appearance on the dorsal shield is practically 

 simultaneous with these organs;' and before the eyes have 

 travelled over the margin tlie free-cheeks must be wholly 

 ventral in position. They are very narrow when first dis- 

 cernible (Plate III, figures 6, 9, and 10), and in Ptyclioparia^ 

 Sao, etc., include the genal angles, but in Dalmanites they 

 extend only a short distance below the eyes. 



The remaining features of the protaspis which here require 

 notice are the pleural furrows and the pygidium. The pleura 

 from the anterior segments of the glabella are occasionally 

 shown, as in the young of Olenellus (figure 81), but usually 

 the pleura of the neck segment are the first and only ones to 

 be distinguished on the cephalon, the others being so inti- 

 mately coalesced as to lose all traces of their individuality. 

 This makes the cranidium, or head shield, exclusive of the 

 free-cheeks, consist of the fused lateral extensions or pleura 

 of the head segments, as already noticed by Bernard. ^^ The 

 possible pleural or segmental nature of the free-cheeks will 

 be noticed later. 



The distinct pleura of the pygidium appear soon after the 

 anaprotaspis stage, and in some genera {Sao, Dalmanites') are 

 even more marked than in the adult state, much resembling 

 separate segments. The growth of the pygidium is very 

 considerable through the protaspis stages. At first it is less 

 than one-third the length of the dorsal shield, but by the 

 successive addition of segments, it soon becomes nearly one- 

 half as long. In some genera it is completed before tlie 

 appearance of the free thoracic segments, though usually new 

 segments are added during the adolescence of the animal. 



A number of genera present adult characters, which agree 

 closely with some of the larval features noticed in this 



