64 THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES. 
is of course a question as to the number and the exact form of those on the pygidium, but 
I think the present restoration is fairly well justified by the specimens. As would be ex- 
pected from the narrow axial lobe, the gnathobases of the coxopodites are short and small. 
Summary on the Ventral Anatomy of Trilobites. 
comparison of appendages of different genera. 
Since the appendages of Triarthrus, Cryptolithus, Neolenus, Calymene, and Ceraurus 
are now known with some degree of completeness, those of Isotelus somewhat less fully, 
and something at least of those of Ptychoparia, Kootenia, and Acidaspis, these forms being 
representatives of all three orders and of seven different families of trilobites, it is of some 
interest to compare the homologous organs of each. 
All in which the various appendages are preserved prove to have a pair of antennules, 
four pairs of biramous limbs on the cephalon, as many pairs of biramous limbs as there 
are segments in the thorax, and a variable number of pairs on the pygidium, with, in the 
case of Neolenus alone, a pair of tactile organs at the posterior end. Each limb, whether 
of cephalon, thorax, or pygidium, consists of a coxopodite, which is attached on its dorsal 
side to the ventral integument and supported by an appendifer, an exopodite, and an endopo- 
dite. The exopodite is setiferous, and the shaft is of variable form, consisting of one, two, 
or numerous segments. The endopodite always has six segments, the distal one armed with 
short movable spines. 
Coxopodite. 
The coxopodite does not correspond to the protopodite of higher Crustacea, the basip- 
odite remaining as a separate entity. The inner end of the coxopodite is prolonged into 
a flattened or cylindrical process, which on the cephalon is more or less modified to assist 
in feeding, and so becomes a gnathobase or gnathite. The inner ends of the coxopodites of 
the thorax and pygidium are also prolonged in a similar fashion, but are generally some- 
what less modified. These organs also undoubtedly assisted in carrying food forward to the 
mouth, but since they probably had other functions as well, I prefer to give them the more 
non-committal name of endobases. 
In Triarthrus and Neolenus the endobases are flattened and taper somewhat toward 
the inward end. In Isotelus, Calymene and Ceraurus, they appear to have been cylindrical. 
In other genera the}' are not yet well known. In all cases, particularly about the mouth, 
they appear to have been directed somewhat backward from the point of attachment. As it 
is supposed that these organs moved freely forward and backward, the position in which they 
occur in the best preserved fossils should indicate something of their natural position when 
muscles were relaxed. 
Cephalon. 
Antennules. — Antennules are known in Triarthrus, Cryptolithus, Neolenus, and Ptycho- 
paria. In all they are long, slender, and composed of numerous segments, which are spinif- 
erous in Neolenus, and very probably so in the other genera. 
In Triarthrus, Neolenus, and Ptychoparia they project ahead of the cephalon, emerg- 
ing quite close together under the front of the glabella, one on either side of the median 
line. In Cryptolithus they turn backward beneath the body, but since only three or four 
specimens are known which retain them, it is possible that other specimens would show 
