238 TRILOBITA CHAP. 
the thorax are the longest; the others gradually decrease in size 
in passing posteriorly. Each thoracic leg (Fig. 142, B) consists 
of a short coxopodite with an inward cylindrical prolongation 
forming a gnathobase which is best developed on the anterior 
legs; the endopodite and exopodite are long and nearly equal ;- 
the former consists of six joints tapering gradually to the end ; 
the latter, of a long proximal joint with a denticulate edge and 
a distal part of ten or more joints, and it bears a row of setae 
along the whole of the posterior edge. 
The anterior appendages of the pygidium differ but little 
from the posterior thoracic legs; but the phyllopodous character, 
which appears in the latter, becomes more distinct in the 
appendages of the pygidium, especially those near its posterior 
end, and is due to the broad, flat, laminar joints of the endopodite. 
The more striking features of the appendages of Z'riarthrus 
are the small amount of differentiation which they show in 
different parts of the body, especially the want of specialisation 
in the cephalic region; the distinctly biramous character of all 
except the first pair; and the presence of one pair of functional 
antennae only, and the occurrence of thoracic gnathobases. 
In 7Zrinucleus the appendages are not so well known, but 
they are considerably shorter than in 77viarthrus. 
In the Palaeozoic rocks of Bohemia, where Trilobites are 
very perfectly preserved, Barrande’ discovered the larval forms 
of several species, and in some cases was able to trace out the 
development very completely, but in others the earliest stages 
were not found. In the strata in which Trilobites oceur Barrande 
found minute spheroidal bodies, usually of a black colour, and 
only a little smaller than the youngest larval stages; those 
bodies are probably the eggs of Trilobites.. Since the publication 
of Barrande’s work the development of some species found in 
North America has been studied by Ford, Matthew, Walcott, 
and Beecher. But even now the development is known in only 
a very small proportion of the total number of genera of Trilo- 
bites. The early larval form (Fig: 145, A) is similar in general 
character in the various species in which it has been found. It 
is circular er ovoid in outline, with a length of from 0°4 to 1 mm., 
and consists of a large cephalic and a small pygidial portion ; 
the axis is distinct and usually shows more or less clear 
1 Syst. Sil. Bohéme, i., 1852, pp. 257-276. 
