Larral Sfofjes of Trifobifes. — Beec/icy. 183 
The genus Triarthrus is taken for the basis oi this restora- 
tion, as it is to-day the best known of all the trilobites, and 
its ventral structure has been ascertained to a degree of per- 
fection of detail which compares favorably with many of the 
recent crustaceans.''- ^''^'•' The writer has studied the structure 
of many adult and immature specimens some of them not more 
than 5 mm. in length, so that fortunately the appendages are 
known at many stages of growth. Especially are the young 
and rudimentary limbs near the extremity of the pygidium 
in adolescent individuals of considerable morphological inter- 
est, for thej' agree closely with the i)hyllopodiform trunk ap- 
pendages in the metanauplius of Apus, and protozoea of En- 
plKuisid, or in a general way, with the still more rudimentary 
trunk limbs in the nauplius stages of these and other forms. 
It has been definitely ascertained that the cephalon in trilo- 
bites bears five pairs of jointed ajipendages or liml)s.'' In lar- 
val or immatvire specimens, and in adults in which the glabella 
retains its primitive structure, this numb<M-is indicated on tiic 
dorsal shield by the five lobes or annulations of the glabella, 
including the neck ring. These may therefore be taken as 
representing, in so far, the original segmentation of the head, 
and agree with what is generally accepted as the primitive 
structui'e in modern true Crustacea. The head portion of the 
protaspis clearl}" shows this j^eiitasomitic structure, and evi- 
dently carried a correspontling number of j^aired liiiil)s on the 
ventral side. It has also been denu)nstrated tliat the auiuila- 
tions on the axis of the i)ygi(liuiii (•orres[)oii(l to the iiiiiiiher 
of paired limbs beneath, exclusive, of course, of the anal seg- 
ment. Here, too, it is possible to tell from the pygidial por- 
tion oi' the i)rotaspis the Diimhcr of limbs pri*S(Mit during life. 
The protaspis of TflarUinis^ represoited in jtlate VIIT. figure 
13. on this basis had five pairs of limbs attached to tlie head 
portion and two pairs to the pygidium. 
Next, as to the composition and form of these elementary 
protaspis limbs, it is safe to assume that the anterior pair, 
corresponding to the aiitciinuN's, must !)<■ iiniranious since 
ihey are so during all the young aiul adult stages observed, 
and since this form is common to all nauplius stages of modern 
Crustacea, and is rec(»gni/,ed as primitive and elementary for 
the class. There is apparently a greatei- siniihirity in tlie 
