ARTHROPODA. TRILOBITA. 205 



Pygidium semicircular, formed of from twelve to eighteen 

 segments, margin entire. Devonian to Permian. 



Griffithides. Body oval ; glabella pyriform without 

 lateral furrows, with inflated basal lobes ; eyes small. 

 Thorax with nine segments. Pygidium rounded, formed 

 of about thirteen segments. Carboniferous. 



Distribution of the Trilobita. 



The Trilobites form one of the most important and 

 striking features in the Palaeozoic faunas, they occur first 

 in the Lower Cambrian Beds, and reach their maximum 

 in the Ordovician ; they are still abundant in the Silurian, 

 but become less important in the Devonian, and in the 

 Carboniferous are represented by four genera only. In 

 Europe they do not extend beyond the Carboniferous 

 Limestone, but in North America one species of Phillipsia 

 has been found in the Permian. 



The most important genera are : 



Cambrian. Agnostus, Microdiscus, Paradoxides, Olenellus, Olenus, 

 Conocoryphe, Niobe, Asaphus, Angelina, 



Ordovician. Ogygia, Ampyx, TrinucUus, Illcmus, Asaphus, ^glina, 

 Calymene. 



Silurian. Phacops, Homalonotus, Calymene, Acidaspis, Cheirurus, 

 Proetus, Illcenus, Lichas. 



Devonian. Phacops, Homalonotus, Cheirurus, Bronteus, 



Carboniferous. Phillipsia, Griffithides, Brachymetopus. 



ORDER. XIPHOSURA. 



The only living representative of the Xiphosura is 

 Limulus, the king-crab, found on the eastern shores of 

 North America and Asia, and in the Malay Archipelago. 

 The body is protected by a chitinous shell, which shows a 

 fairly well marked trilobation ; the cephalothorax is 



