Larval Stages of Trilobites. — Beecher. 179 
ally found in the glabella? of some species. They do not indi- 
<'ate an additional pair of limbs, for the extra lobe is produced 
((/) by division of the anterior lobe through the greater or less 
•extent of the eye-line across the axis, as in Olenelhis. Paradox- 
ides and Ogiigia : or (6) by the marked development of mus- 
cular fulcra, which are supposed to be connected with the 
hypostoraa. 
The next structures not especially noticeable in all stages 
of the protaspis are the free-cheeks, which usuall}^ 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 simultan- 
eous with these organs; and before the eyes have travelled 
over the margin, the free-cheeks must be wholly ventral in posi- 
tion. They are very narrow when first discernable (plate VIII, 
figures 6, 9 and 10), and in Piitchoparia^ Sao, etc., include the 
genal angles, but in Dalmanites they extend only a short dis- 
tance 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 Olenelliis (figure 6), but usually the 
pleura of the neck segment are the first and only ones to be 
distinguished on the cephalon, the others being so intimately 
coalesced as to lose all traces of their individuality. This 
makes the cranidium, or head shield, exclusive of the free- 
<;heeks, consist of the fused lateral extensions or pleura of the 
head segments, as already noticed by Bernard. i- The possible 
pleural or segmental nature of the free-cheeks will be noticed 
later. 
The distinct pleura of the pygidium ai)i)ear soon after the 
anaprotaspis stage, and in some genera [Sao, Dalmanitea) are 
even more marked than in the adult state, much resembling 
separate segments. The growth of the pygidium is very con- 
siderable 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 the appearance of tlie 
free thoracic segments, though usually new segments areadd- 
«d during the adolescence of the animal. 
