310 CRUSTACEA. TRILOBITA 



segments, pleurae with ridges produced into long spines. Pygidium 

 small, with long spines. Llandeilo Beds to Devonian. Ex. A. 

 barrandei, A. brighti, Wenlock. 



Phillipsia. Body oval ; glabella with nearly parallel sides, 

 with 3 or 4 narrow lateral furrows, of which the posterior one 

 curves backwards and joins the deep neck-furrow, thus cutting 

 off a basal lobe. Facial sutures cut the posterior border obliquely, 

 and the anterior border in front of the eye. Free cheeks large ; eyes 

 large, reniform. Thorax with 9 segments, pleurae grooved. Py- 

 gidium semicircular, with 12 to 18 segments, margin entire. De- 

 vonian to Permian. Ex. P. derbiensis, Carboniferous. 



ProetUS. Closely allied to Phillipsia but with fewer segments 

 in the pygidium. Ordovician to Carboniferous, chiefly Devonian. 

 Ex. P.fletcheri, Wenlock. 



Griffithides. Body oval ; glabella with inflated basal lobes 

 cut off by the posterior furrow, and without other lateral furrows ; 

 main part of glabella pyriform ; eyes rather small. Thorax with 9 

 segments. Pygidium rounded, with about 13 segments. Car- 

 boniferous Limestone. Ex. O. seminiferus. 



Distribution of the Trilobita 



The Trilobites are confined to the Palaeozoic period, 

 and form one of the most important and striking features 

 in the faunas of the Lower Palaeozoic deposits. They 

 occur first in the Lower Cambrian Beds, and reach their 

 maximum in the Ordovician. In the Silurian, Trilobites 

 are still abundant, 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. 



Already in the Cambrian period the Trilobites were 

 represented by a considerable variety of forms, showing 

 that even then the group must have been of considerable 



