Larval Stuyes of Tj'/lobife^. — Bccclicr. 173 
and the glabella does not expand and terminate in the ante- 
rior margin. No eye-line is present, but the eye-lobes may be 
seen a little within the margin. The glabella has the charac- 
teristic number of annulations; margin provided with a row 
of denticles; genal angles extended into spines; ])ygidium 
with four spines. 
The adult condition (figure 3) shows that the eyes have 
moved inwards and backwards to near the neck segment. The 
glabella has lost its annulations and is broken up into a me- 
dian lobe with two smaller ones on each side, while the neck 
ring is projected into a spine. The changes noted here are 
much more profound than in any of the preceding genera, 
since Acida^fpis is one of the most highly specialized of trilo- 
bites in its glabellar structure and elaborate ornamentation. 
The protaspis, too, partakes of this specialization, and, al- 
though the general form of the shield and the annulation of 
the axis are as primitive as in Triurfhrus, yet the character- 
istic spinosity of the genus appears even at this early stage 
and is a marked instance of acceleration of development. 
Arges consaiigiiineiis Clarke; plate IX, Hgure 4; from the 
Lower Helderberg group, Albany county. New York. A sin- 
gle larval form of this type has been found and at first was 
provisionally referred to Fh(('efh(>)iides.^ The recent publica- 
tion by Clarke,^* of Arges consa ngiu'iiens from the same horizon, 
and a comparison of the larva with the description and with 
considerable additional material, renders it now possible to 
determine definitely the relations of this interesting form. As 
the main details of structure in Acidasplx and Arges are so 
similar, the transformations undergone by the larva are much 
alike in each case. The young Arge.s likewise shows the same 
acceleration in the development of the spines and surface or- 
namentation, and the retention of the primitive features of 
the glabella. The specimi^n sesn in figur? 4 repres-Mits a late 
larval stage (paraprotaspis), as shown by th;> transverse form 
of the cephalon and the larg.^ size of th.^ pygidium. 
Pro'etKx iKirrliisciiliis Hall; plate IX, figures 5, 6 and 7 ; 
Utiea slate, near RoiU', New York. Two larval stages of this 
species have been found. The ^''ounger (figure 5) is smooth, 
broadly ovate, .72 mm. long, antl widest in front; axis dis- 
tinctly annulated. cylindrical on the cephalon, tapering on the 
