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1903. ] PACKARD—CLASSIFICATION OF ARTHROPODA. 149 
bita that they have no relationship with the Crustacea. To include 
them in that group, otherwise a mos: natural one, is not good tax- 
onomy. The chief characters which are given for retaining the 
Trilobita as a primitive group of Crustacea are the presence of the 
antenne-like preoral appendages of Triarthrus and one or two 
other genera. That this is not so important as might seem at first 
sight is the presence of four antennz-like appendages in the head 
of Eurypterus; Holm having discovered that the first pair are 
chelate, like those of Limulus. 
Both Trilobita and Crustacea have biramose limbs, adapted for 
swimming, but so nas any marine arthropod; the fact that the 
limbs are divided is the result of their inheritance in either class 
from Annelids with parapodia, but in the multiarticulate structure 
of each ramus and the entire lack of differentiation of the whole 
series of postoral limbs in Triarthrus we have fundamental charac- 
ters which are diagnostic of the Trilobites, and widely separate the 
class from the Crustacea. Whether the Merostomata are widely 
distinct from the Trilobita or not, we submit that it is a mistake to 
include the latter in the class of Crustacea. 
Entirely disagreeing with those who widely separate the Merosto- 
mata from the Trilobites, after repeatedly going over the subject, the 
close relationship of the two groups seems to us to be very apparent, 
the differences being only such as would separate the two classes of 
a single phylum. It has seemed to us that the merostomes and 
trilobites either had a common ancestry, which was a protaspid, or 
the Merostomes by way of the Synxiphosura diverged from the 
trilobite stem after it had been established in Precambrian times. 
Thus far, unfortunately, we know nothing of the nepionic stage of 
any of the Eurypterida. Their earliest adult form (Strabops of 
Beecher!) occurs in the Cambrian, while the Synxiphosura date 
from the Silurian. It is not improbable that some genus of this 
group gave origin to the Xiphosura. On the other hand, is there 
not so close a resemblance between some of these Synxiphosura, 
such as Neolimulus and Bunodes, as to suggest that the Merostomes 
are direct descendants of the Trilobita? If we compare the figures 
1 Although Beecher refers this early form to the Eurypterida, it appears, judg- 
ing from his figure, to quite as much resemble certain Synxiphosura, as Bunodes 
and Neolimulus, in the short, broad head and shape of the trunk-segment and 
telson, though it has two segments more than in the Synxiphosura and one seg- 
ment less than in the Eurypterida. 
