TRILOBITA. 363 



ment, the number of generic and specific types already 

 known from this formation being very large, while indi- 

 viduals are sometimes extremely abundant. Some of these 

 so-called " Primordial " Trilobites attain a very great size, 

 being only surpassed amongst later forms by some species 

 of Asaphm. They mostly belong to the families of the 

 Agnostidce, Conocephalidce, and Paradoxidce, and to the genera 

 Paradoxides, Conocoryphe, Sao, Ellipsocephahis, Hydrocephahis, 

 Arionellus, Dikellocephalus, &c. Along with these, however, 

 are genera such as Agnostus and Olenus, which pass up into 

 the Silurian. Some of the " Primordial " Trilobites are not 

 so highly organised as their successors, as shown by the 

 occasional absence of eyes, or want of the facial suture, by 

 the imperfect segmentation of the glabella, or by the multi- 

 plication or diminution of the number of the body-rings ; 

 but others do not exhibit any inferiority to those of later 

 periods. 



In the Lower and Upper Silurian rocks the Trilobites 

 attain their maximum of development, the leading families 

 being the Asaphidce, Phacopidce, Trinucleidce, Cheiruridw, 

 and Calymenidce, and the chief genera being Asaphus, Ogygia, 

 Phacops, Trinucleus, Ampyx, Clieirurus, Encrinurus, Caly- 

 mene, and Homalonotus. In the Devonian rocks, again, 

 Trilobites are tolerably abundant, though less so than in 

 the preceding series. The commonest Devonian genera are 

 Phacops, Homalonotus, and Bronteus. Lastly, the order 

 seems to die out before the close of the Carboniferous 

 period, being represented in the Carboniferous period solely 

 by the four genera, Phillipsia, Brachymetopus, Griffithides, 

 and Proetus. 



In the following, we shall give a brief summary of the 

 families of the Trilobita, indicating the principal genera of 

 each, and their distribution in time. No strictly zoological 

 arrangement of these families is as yet possible, except in a 

 general sense, but we have, in the main, adhered to the 

 classification proposed by M. Barrande, one of the most 

 illustrious palaeontologists of the age. 



1. HAKPEDID^E. Cephalic shield large, and horse-shoe- 

 shaped, its posterior angles greatly prolonged, and its mar- 



