RAYMOND: NEW AND OLD SILURIAN TRILOBITES. 39 
There is still a third possibility, namely, that Ceraurus hydei is the 
young of Cheirurus niagarensis, with which it occurs. On this third 
point, Weller states that the fixed cheeks of C. hydez lack the pitted 
surface characterizing C. niagarensis, and that C. hydei has a border 
all around the cephalon, while C. niagarensis lacks it in front of the 
glabella. These facts seem to be borne out by the type which is now 
before me, and Professor Weller might have added that the glabella 
expand more rapidly in the young of C. niagarensis than in C. hydet, 
and has deeper glabellar furrows. The eyes too, of the young of C. 
niagarensis are much further back than those of C. hydev. 
Against these differences we may, however, place the fact that the 
thorax is alike in the two species, and more similar to the thorax of 
Cheirurus insignis Beyrich than to any of the Ordovician species of 
Ceraurus. In both Cheirurus niagarensis and Ceraurus hydei, the 
part of each pleural lobe between the dorsal furrow and the fulcral 
line is very much reduced, the diagonal furrow is very short, and the 
two small nodes which it separates are narrow, and one directly in 
front of the other, a point not brought out in Weller’s somewhat 
generalized figure. On the fulcral line there is a row of nodes, and 
just inside this row is a longitudinal furrow parallel to the dorsal 
furrows. Beyond the fulcral line, the pleura are free, not contiguous 
as shown in Weller’s figure. These same characteristics are shown in 
two specimens of Cheirurus niagarensis from Wauwatosa, Wisconsin. 
On the whole, it does not seem very probable that C. hydez is the 
young of Cheirurus niagarensis, especially as there is another species, 
Ceraurus nuperus (Billings) which has a Cheirurus-like cephalon and 
Ceraurus-like pygidium. The choice seems to lie between calling 
it a Ceraurus or a Cheirurus. Theoretically, it would seem that the 
Ceraurus pygidium was more specialized, and, therefore, less apt to 
be duplicated than the Cheirurus head. Most of the other Cheiruri- 
dae, except Ceraurus, ‘have all the spines of the pygidium approxi- 
mately equal. 
In Ceraurus pleureranthemus there is a tendency in some specimens 
to have the basal lobes of the glabella triangular instead of square, 
and in Ceraurus misneri the glabella occupies a large part of the 
cephalon, and the cephalon is long. Further, Ceraurus reaches the 
climax of size and abundance in the Trenton, the late species being 
smaller, and the specimens rarer. As to the thorax, I have shown that 
this portion of the test changes in parallel directions in many lines of 
the Asaphidae, and the same might well happen in the Cheirurudae. 
On the other hand, one would not expect a decadent race to show new 
characters similar to those of a race which is at its best. 
