34 The American Geoloijist. Jui,v,i«97 
to regard theiu as most nearly related to the modern phyllo- 
pods, although in his latest he is inclined to assign them the 
rank of a sub-class, equivalent to Entomostraca and Malacos- 
traca. None of these suggestions, except the last, commends 
itself to me. 
In the first place, as opposed to Bernard's view, all existing 
evidence goes to show that the trilobites are truly Crustacea; 
the Xiphosures are as clearly nothing of the sort. Not a sin- 
gle homology can be drawn with anj'- degree of certainty be- 
tween Limulus and any trilobite. The regional divisions 
are different; the appendages are built upon a different plan, 
while the larvae of the two groups present but the slightest 
and most superficial resemblance to each other. Certainly, if 
Beecher's suggestion that the larval (Protaspis) stage of the 
trilobites represents the ancestor of the nauplius be correct, 
then the larvae must be totally distinct, for Limulus shows 
nothing in its development in the least approximating a nau- 
plius stage. 
But is this Protaspis stage possessed of nauplius character- 
istics? Is it to be regarded as a "protonauplius" ? I think 
not. The characteristics of the crustacean nauplius are a 
(usually) unsegmented body, bearing a single median ocellus 
and three pairs of appendages. Of these the first is pre-oral 
and unbranched, the other two are biramous, and have their 
basal joints modified for mastication. The mouth is on the 
under surface ; in front of it is the large upper lip. The vent 
is terminal and dorsal in position. Certain of these features 
are characteristic, not only of the nauplius, but of all metameric 
invertebrates as well. 
In the larvffi of the trilobites median and pleural regions 
are differentiated, a condition, so far as I am aware, not par- 
alleled in any nauplius and certainly not to be regarded as 
"protonauplian." No traces of the median eye have been 
found in any trilobite larvae, a matter of no little importance. 
The trilobite larvae, on the other hand, frequently possess 
paired eyes, a condition unknown in any nauplius. The ce- 
phalic region of the trilobite larva is segmented; in no nau- 
plius is this the case. Further we know as yet absolutely 
nothing of the appendages of the youngest trilobite larvae, nor 
of the characteristics of the upper lip. Beecher has restored 
