Systematic Position of the Trilohites. — KirKjsley. 35 
tliese portions with some plausibility in his account ('95) of 
the Protaspis stage, but I cannot help thinking that he may 
be in error here since he seems in all of his papers to regard 
the biramous structure of the crustacean appendage as the 
primitive condition, a supposition negatived by the thorough 
researches of Lankester ('81) upon the crustacean foot, and 
the confirmation of this view which we owe to Packard ('83). 
It should also be remembered that the great majority of em- 
bryologists agree in refusing to the nauplius much phyloge- 
netic significance, holding rather to the view that its charac- 
teristics are adaptive rather than phylogenetic. 
Dr. Beecher also places much weight upon the existence of 
five segments in the head region of the trilobites, as evi- 
denced not only by the annulations upon the dorsal sur- 
face but by the appendages on the ventral surface as well. 
These he seems to regard as tending to show that the head in 
these ancient forms agrees "with what is generally accepted 
as the primitive [head] structure in modern true Crustacea." 
But this at once leads into many difficulties. Setting aside 
the fact that there seems to be no general agreement as to 
what constitutes the "head" in modern Crustacea* we are at 
once met with the fact that either the antenn;e or the anten- 
nuhe are apparently unrepresented in the trilobite head. This 
involves more than a dilemma, for it has several horns. 
First, we ma}- assume that in the trilobites. as in some phyllo- 
pods and isopods, one pair of antennae has been so reduced that 
it is no longer recognizable in the fossils. But with this assump- 
tion the conception of the trilobite head at once changes ; it is 
no longer composed of five but of six somites. f On the other 
hand if we assume that there has been no dropping out of ap- 
pendages, and proceed upon the basis of exact serial homology, 
then there comes the difficulty that the second pair of antennje 
*What, for instance, shall we recognize as typical for the crustacean 
head? In the Tetradecapoda we have a "head'" with six pairsof appen- 
dages. In the Decapoda we can differentiate two kinds of "head" ac- 
cording as we recognize the mouthparts as cephalic ajjpendages (eight 
segments) or as we take the cervical suture as constituting the bounda- 
ry between head and thorax, a line of division which would give a head 
of only two segments (cf. Ayers, '85). Again the Phyliopoda give a 
head of three to six segments, according as we draw the line. 
tOf course nothing can be said of the true segments of the head as 
indicated by the ueuromeres, etc., as can be done with the embryos of 
existing forms. (Cf. Bumpus, '91, McMurrich, '95.) 
