408 GEOLOGICAL SURVEY OF THE TERRITORIES. 
and an outer or respiratory portion, as in Nebalia and Decapoda. The 
endopodite of Limulus (en) is axially jointed, there being three well- 
marked joints to this part of the limb. The branchiate portion of the 
limb (ev) is homologous with that of Nebalia, and the epipodital or 
branchiate portion of the Decapod thoracic hmb. At the same time 
that of Limulus presents some remarkable peculiarities, 7. ¢., the exo- 
podal (or epipodital) portion is jointed; and the gill, instead of being a 
simple fan-like extension, as in the Phyllopoda and Phyllocarida, is re- 
placed by a number of flat, thin gill-plates, arranged parallel to each 
other, in an antero-posterior sense. When, however, we compare the 
gill, or rather the epipodital portion of the leg of Limulus, with that of 
the lobster we have the various fundamental elements, 7. ¢., an artery 
and a vein passing into the foot and in connection with a number of 
gill-plates. In the lobster we have along the base of the gill (fig. 33) 
collective veins and an artery into which the blood passes after being 
aerated in a large number of cylindrical gill-filaments. Morpholog- 
ically there is a fundamental resemblance between the two types of 
branchiz; in Limulus there are gill-plates, in Decapods gill-filaments, 
each presenting in the aggregate a large respiratory surface. The gills 
of the Isopoda are in some degree intermediate between the Decapods 
and the Merostomata. 
When we compare the anterior or cephalic appendages with the 
thoracic appendages of the lobster, there is a close resemblance in the 
axially-jointed endopodite (fig. 38, end) of Limulus with its large terminal 
claw to the foot of the Decapod. The absence of the gill or branchiate 
(epipodital) portion in Limulus is correlated with the ambulatory nat- 
ure of its anterior or cephalic appendages. 
In the trilobites, however, as may be seen by Mr. Walcott’s able res- 
toration (fig. 40), we have attached to the thoracic ambulatory feet 
a respiratory epipodital portion. In some respects, then, in the trilo- 
bites we have a style of structure intermediate between the Merosto- 
mata and the Decapoda. 
In the trilobite we apparently have, besides a true-jointed locomotive 
endopodite (fig. 40, en), an inner jointed appendage (en’), which may 
be homologized with the exopodite of the Decapod maxillipede (fig. 
33). From near its base arises the two singular spiral gills, which are 
unique. It is to be observed that the two jointed appendages and the 
stem of the gills arise from what appears to be a true coxopodite, and 
that this coxopodite is apparently homologous with that of Limulus 
(fig. 38). It thus appears that a study of the general internal anat- 
omy and of the appendages of the normal, recent Crustacea (Neocarida) 
throws light upon the structure of the archaic Crustacea (Palwocarides), 
and that the most archaic Neocarida, the Phyllocarida (Nebalia), as re- 
gards their thoracic limbs, do not remotely resemble the abdominal 
limbs of Limulus. In this connection we would draw attention to fig. 
39, which is designed to show the possible relations between Limulus 
and Calymene or the Merostomata and the Trilobita. The essential 
difference is in the nature of the limbs; the thoracic limbs of the trilo- 
bite, while having a jointed endopodite as in Limulus, also having an 
exopodite and a forked spiral gill. Now, if we append to the coxopo- 
dite of Limulus an exopodite, and instead of having the gills arranged 
anteroposteriorly, like the leaves of a book, have them arranged on one 
side (the outer) of a more or less cylindrical epipodite, as we have 
drawn them in fig. 39, we shall, hardly be doing greater violence to 
nature than we see to occur in any Decapod, where, as may be seen in 
fig. 35 of the lobster, the maxille have no specialized exopodite, such 
