172 The American Geoloijht. Septcnibcr, 1895 
the neck segment and those of the pygidium. The next stage 
(figure 10) probably re])resents the close of the protaspis stage 
(paraprotaspis) and the inception of the nepionic condition, 
when the ce])halon and pygidium are distinct and before the 
development of the free thoracic segments. 
In considering the changes necessarily passed through by 
these larvffi previous to attaining their adult characters (figure 
11) the most notable, aside from increase in size and addition 
of the sixteen thoracic segments, are : the appearance and 
translation of the eyes pm'i passu with the growth of the free- 
cheeks ; the growth of the border in front of the glabella, 
which now narrows anteriorly, and terminates abovit one-third 
the length of the cephalon within the margin ; the less dis- 
tinct annulation of the glabella ; and the development of the 
spines and tubercles ornamenting the test. 
Triarthrvs beck/' Green; plate VIII, figures 12, 18, and 14; 
from the Ordovician, Utica slate, near Rome, N. Y. A larval 
form of this species was figured by the writer^ in 1893. At 
this time, the eye-line was confused with the anterior annula- 
tion of the axis, making the cephalon appear to have six in- 
stead of five annvilations. A recent examination of a large 
number of specimens shows that five is the invariable number, 
as here represented. Two protaspidian stages of this species 
have been noticed, dilfering chiefly in the size of the pygidium. 
Both agree in showing a strongly annulated axis, not ex- 
panded in front and terminating some distance within the 
margin. From the first annulation, a slightly elevated ridge 
on each side indicates the eye-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. 
Aci'dasiJi's tiiberci'I((fa Conrad; plate IX, figures 1, 2 and 8; 
from the Lower Helderberg group, Albany county. New York.^ 
Several of these remarkable larvie have been found perfectly 
silicified in a limestone from which they have been freed by 
etching. In general form, they resemble the second larval 
stage of iSao (plate VIII, figure 9), but the pygidium is shorter 
