362 SYNOPTIC COLLECTION. 



In an older protaspis (fig. 2, x 30) the central axis is 

 segmented ; the pygidium has developed, and is distinctly 

 segmented. The free cheeks, though narrow, are at the 

 margin so that they can be seen in a dorsal view (fig. 2). 

 In a still older protaspis (fig. 3, x 30) the pygidium is 

 complete but the thoracic segments are not yet formed. 

 When the protaspis stage has passed into the nepionic 

 stage the eyes and free cheeks have migrated to the dor- 

 sal side of the cephalon (fig. 4, cephalon, pygidium not 

 drawn. The shaded parts are the free cheeks; the cres- 

 cent eyes are seen at the ends of the eye-lines). 



The thoracic segments form between the cephalon and 

 pygidium, as seen in the adult (fig. 6, x ). They are 

 freely movable, while those of the caudal shield are con- 

 solidated. The free cheeks become larger and the eyes 

 are farther from the margin (fig. 5). The central portion 

 of the head region, or in other words, the forward part of 

 the axis, is known as the glabella, and the fixed cheeks 

 are situated between this part and the free cheeks. Other 

 characters of the adult trilobite are better seen in Triar- 

 thrus, the next genus to be described. 



The protaspis of Triarthrus differs somewhat from that 

 of Sao, since the central axis of the cephalon does not 

 extend to the anterior edge (PI. 888, fig. i), and the eye- 

 lines run from the first segment to the margin. A resto- 

 ration of the ventral sida of the protaspis at this time is 

 represented in fig. 2. Since the head region has five 

 segments it is inferred that it has as many pairs of ap- 

 pendages, and that the pygidium has two pairs for the 

 same reason. The first pair of appendages are un- 

 branched and are probably sense organs, but the remain- 

 ing pairs are two-branched and adapted for swimming. 

 The segments of the pygidium increase in number, as 

 seen in fig. 3. 



The adult (No. 889 ; Nos. 890, 891, models ; also PL 

 888, fig. 4) has a small cephalon, while the thoracic and 

 abdominal regions are divided into distinct segments (No. 



