The Ventral Structure of Triarthrus. — Beecher. 99 
dous appendages (8, 9). The general similarit3^ of the ce- 
phalic organs of Apus and 7'/'/rf>-('/?/'»s is quite apparent. The 
most conspicuous differences, as the absence of normal endo- 
podial and exopodial elements, disappear in a stud}" of the 
ontogeny of the limbs of J/>/^s■, thus bringing these organs in 
the two groups into nearly exact correlation. 
There are, however, important structural features of other 
parts of the bod}", which are quite dissimilar from Api>s 
and the higher Crustacea, and the exact relations of the trilo- 
bite with any one group cannot be considered as fixed. Points 
of likeness may be established with almost every order, show- 
ing chiefl}' the relationship between the trilobite and the an- 
cestors of modern Crustacea. 
SiiiiDnanj of central oryans of Tririrthn/.s. 
A pair of appendages to each potential segment of the 
animal, all of which are biramous except the anterior pair. 
Five pairs of appendages on the cephalon. 
Anterior antennie, or antennules, attached at the sides of 
the hypostoma; simple, with a single manj^-jointed Hagellum. 
First pair of biramous limbs, or posterior antennie, with 
endopodite and exopodite ; basal portion manducatorj'^ in 
function. 
Second pair of biramous limbs, or mandibles, similar to 
preceding. 
Third and fourth pairs of biramous limbs, or maxilhe, same 
as preceding, with large gnatliobases, well-developed endop- 
odites and fringed exopodites. 
Thoracic limbs biramous; endopodite a jointed crawling 
leg; posteriorly the joints become flattened and leaf-like: 
exopodite fringed with seta% and developed into a swimming 
organ; coxopodite with gnathobase. 
Appendages of the pygidium, true phyllopodous limbs. 
Hypostoma. 
Mouth between hypostoma and metastoma. 
Metastoma, a convex crescentic plate, witii sick' hippets. 
Anus in median line, near ventral extremity of pygidium. 
EXPLANATION OF PLATES. 
I'l.ATK IV. 
TKIAHTIIKIIS IJKCKI Grct'll. 
Fijiurc 1. — N'ciiU'iil side of a ljii"f!:<' s|)fcini<'ii: slmwiii;; al each sidi' of llic 
liyposloina llu' aiilfiiimlcs. witli tlicir siti^'lc Hairrlltiin and stroiiii' 
