36 The American Geologist. July, is97 
of tlie true Crustacea are equivalent to what in the trilobites is 
regarded as the mandible, and we are at once forced to divide 
the whole crustacean phylum into two classes, the one contain- 
ing the trilobites, and the other those forms which are undoubt- 
edly crustacean, a division which seems to me warranted by 
many other facts of structure and which I have already a- 
dopted ('94) and which Dr. Beecher is also inclined to accept 
('97, pp. 93, 94). 
There is even less evidence to support the supposed phyllop- 
odan affinities of the trilobites. The typical phyllopodan 
appendage has already been well described by both Lankester 
and Packard,* and there is nothing biranious about it, while 
there is much plausibility and even probability in the view 
that it represents the ancestral crustacean appendage, and is 
itself a direct derivative of the annelid parapodium. In the 
trilobites, on the other hand, the appendages, as shown (anten- 
nae excepted) in the restorations of both Walcott and Beecher, 
are as truly schizopodal as those of any Mysis, and they ex- 
hibit in these restorations not the slightest approach to the 
phyllopodan type. 
It is true that Beecher has alreadj^ figured, side by side ('94, 
pi. VII, figs, 3 and 4) the appendages of trilobites and a lar- 
val Apus, the intention being to illustrate the similarity of 
form. It however seems to me that this comparison is unten- 
able since it assumes that the endites of the phyllopod appen- 
dage are comparable to the separate articles of the so-called 
typical crustacean limb, a view which Lankester sixteen years 
ago showed to be without a true morphological basis. 
In the trilobites the pygidial region (why not abdomen?) 
bears appendages serially similar to those of the thorax; in 
the phyllopods, on the other hand, this region in both young 
and adult is without limbs, aside from the caudal furca. In 
the trilobite the body is divisible into axial and pleural re- 
gions, and the persistence of this characteristic in all the gen- 
era with which I am acquainted gives it a greater importance 
than might otherwise be attributed to it. In the phyllopods, 
on the other hand, nothing of the kind is found; thecarapax 
of the Apodidse and the bivalve shell of the estherians being 
*In the few points where Lankester and Packard disagree, I accept 
the interpretations of the former. 
