98 The American Geoloijist. February, i895 
several observations uj)on its general organization and com- 
parisons with other Crustacea may l)e made, This cannot be 
done exhaustively or comprehensive!}^ at this time, and only a 
lew points will be touched upon. The simplicity and primi- 
tiveness of the trilobite structure will first impress the student. 
The variable number of segments in the thorax and pygidium 
in the different genera shows the unstable metameric condi- 
tion of the class. The head alone seems to have a permanent 
number of segments and appendages. Eveii this is not often 
apparent, but the constant number of five head segments in 
larval trilobites shows this to be the true number, although 
subsequent growth may obscure or obliterate this pentasomitic 
character, as has been shown by the writer in Acidaspis (Am. 
Jour. Sci., Aug., 1893) and observed in other genera. 
With the exception of the antennules, all other paired ap- 
pendages of the animal seem to agree in every point of struc- 
ture, and vary only in the relative develoi)nient of certain 
parts. The appendages of the pygidium are ontogenetically 
tlie youngest, and express the typical phyllopodiform struc- 
ture. Passing anteriorly, the joints become less leaf-like, un- 
til in the anterior thoracic legs thej- are quite slender, and the 
limbs resemble those of schizopods. Corresponding to this, 
there is, through the whole series, a gradual development of a 
process from the coxopodite, forming a gnathobase to the 
limb. On the head, these serve as true manducatory organs. 
Posteriorly, they were like the basal endites of Apus^ and en- 
abled the trilobite to convey food along the entire length of the 
axis to the mouth. 
Bernard (1. c.) has nuide a strong exposition of the ev- 
idence in favor of the phyllopod attinities of the Trilo- 
bita, and especially of their relations to Apiis. A por- 
tion of the under side of the head of Apxs is introduced 
for comparison on plate v, figure 12. Both pairs of an- 
tennal organs (1, 1) are rather rudimentary^ in this genus, 
and are situated further forward than in TriarUirns. The 
powerful mandibles (8) are partly covered by the labruui, or 
hypostoma {hn). Then follow two well-developed gnatho- 
bases, representing the maxilhe (4, n), the more slender max- 
illiped (6) and the large first thoracic limbs (7), behind which 
are the basal endites. or gnathobases, of two of the phyllopo- 
