184 The Ahiericatl GeohKjixf. September, 1H95 
larval anteniiuU'S tliaii Ix-twctMi any otlicr apixMuiages, and as 
Apus and Knjilxmsid lia\c tlu-sc in a very ^'cncralizcd t'drni, 
they ai*e taken as types of the first ])air of limits of the trilo- 
bite protaspis, as sliown in plate X, figure I (I). It siiouUl 
be noted, too, that the antenniiles of the triU)bites arist- from 
the sides of tiie up])er li]) or hypostoma, as in thenauplius. 
The other head appendages are typieally branched, though 
in many of the recent Criistaeea they h)se this character after 
the larval stages. Especially is tiiis true of tlie tliird pair of 
limbs, which l)econie modified into the mandibles. In trilo- 
bites the primitive biranious structure of the head limbs per- 
sists to adult stages, occurring also in limbs of all the posterior 
segments where they become more and more phyllopodiform.'^ 
In the restoration of the protaspis it seems only necessary to 
append this archaic ty|)e of limb to each segment, agreeing as 
it does in form and structure with tlie rudimentary limbs of 
older stages and with the nau])lius and metanau]ilius stages 
of Apt',s-. 
It cannot be doubted that the prf»tas]>is had five [)airs f)f 
limbs on the head portion and one or more on the pygidium, 
and although these are the main ])oints necessary to prove the 
argument in the next section, on the nau])lius. yet it seems 
perfectly wai'rantable and better for graphic purposes to at- 
tach the required number of elementary limbs to tiie ventral 
side of the protaspis, as represented in plate X, figure 1. 
There are other organs and structural details occurring in 
the nauplius and in adult trilobites, which deserve recogni- 
tion in a I'estoration of the protaspis stage. First among these 
is the labrum, or upper li]). Nowhere is this |)late so well 
developed and so striking a ventral feature as among tlie tril- 
obites. There can be no hesitation, therefore, in accepting 
this as characteristic of the protaspis. 
The trilobites and most recent crustaceans have a metas- 
toma, or lower lij). This is already developed in the nauplius 
stage of some Crustacea, as Kiiitha iixia and Peiieii.s, and prob- 
ably represents an early larval character. It usually appears 
as a median plate divitled into two small plates, or lappets, on 
each side of the median line, posterior to the mouth, and is 
thus represented in the restored protaspis. As it occurs on a 
segment bearing also a pair of legs and has no separate neu- 
romere, it cannot well be considered as I'epresenting a somite. 
