238 TRILOBITA 



CHAP. 



the thorax are the longest ; the others gradually decrease in si/.c 

 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 Triartkrus 

 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 Trinucleus the appendages are not so well known, but 

 they are considerably shorter than in Triarthrus. 



In the Palaeozoic rocks of Bohemia, where Trilobites are 

 very perfectly preserved, Barrande J 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 occur 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, 

 ;md Beecher. But even now the development is known in only 

 a very small proportion of the total number of genera of Trilo- 

 t'ites. The early larval form (Fig. 143, A) is similar in general 

 character in the various species in which it has been found. It 

 is circular or 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. Sll. Bohemc, i., 1852, pp. 257-276. 



