SILURIAN ROCKS. 



91 



compound, and the individual can be - 



with a lens, and although g 



gether and raised in genus 



Pha metimes to be s 



ich of th the ceph< 



n the genus Trinucleus. The intestine 

 ' ■■ - s • - I I >n the under side oi the 

 developed. 



:nue, there were 



., but only one of them 



'•d. The jointed 



limbs on the trunk c 



- 

 : the other 1 gill; so 

 that tin- ar to tx 



to the h 

 Crustacea tern. 



s, notwithstanding their 

 irkable external resem- 

 blance to the k;: g 



. they approxi- 

 mate. 



The 1:: adually di- 



minish in size from front to 

 back, where the bin 



ind rudiment 



The two longitudinal 

 grooves make the three 

 lobes of a Trilol 



The m a ent Trilo- 



bites include the simplest and some of the m 

 complex. They vary chiefly in the number of 

 :he form and size of the cephalo-tho- 

 rax, and the pygidium; in the elongation of the 

 pleurae of the abdomen ; in the external ornament 

 of the carapace, which sometimes takes the form 



Triarthrus, 



Fig. 11. — A Trilobite show- 

 ing feet and antennae. 



