LARVAL STAGES OF TRILOBITES 173 



the more pronounced annulation of the glabella, together 

 with pleural grooves from the neck segment and those of the 

 pygidium. The next stage (figure 10) probably represents 

 the close of the protaspis stage (paraprotaspis) and the 

 inception of the nepionic condition, when the cephalon and 

 pygidium are distinct and before the development of the free 

 thoracic segments. 



In considering the changes necessarily passed through by 

 these larvre previous to attaining their adult characters (fig- 

 ure 11) the most notable, aside from increase in size and 

 addition of the sixteen thoracic segments, are : the appearance 

 and translation of the eyes pari passu with the growth of the 

 free-cheeks ; the growth of the border in front of the glabella, 

 which now narrows anteriorly, and terminates about one- 

 third the length of the cephalon within the margin ; the less 

 distinct annulation of the glabella ; and the development of 

 the spines and tubercles ornamenting the test. 



Triarthrus Becki Green; Plate III, figures 12, 13, and 14; 

 from the Ordovician, Utica slate, near Rome, New York. A 

 larval form of this species was figured by the writer ^ in 1893. 

 At this time the eye-line was confused with the anterior an- 

 nulation of the axis, making the cephalon appear to have six 

 instead of five annulations. A recent examination ofa large 

 number of specimens shows that five is the invariable number, 

 as here represented. Two protaspidian stages of this species 

 have been noticed, differing chiefly in the size of the pygid- 

 ium. Both agree in showing a strongly annulated axis, not 

 expanded in front and terminating some distance within the 

 margin. From the first annulation a slightly elevated ridge 

 on each side indicates the e3'e-line, and extends to the mar- 

 ginal eye-lobe. The adult form (figure 14) shows, in addition 

 to several characters noted in the previous species, the nearly 

 complete loss of the two anterior annulations of the glabella ; 

 the disappearance of the eye-line; and the development of 

 a row of nodes along the axis, from the neck segment to the 

 proximal segment of the pygidium. 



Acidaspis tuheroulata Conrad ; Plate IV, figures 1, 2, and 



