THE TRILOBITA AND OTHER ARTHROPODA 211 



bend at the eye ; pleura of free somites spiny ; pygidium 

 small, of three to five somites, with pleura projecting 

 from margin. This family includes several remarkable 

 pelagic forms Sphcerexochus (Ord.-Sil.)j Staurocephalus 

 (Ord.), Deiphon (Sil.). The Phacopidce are remarkable 

 for their highly-developed eyes, with many well-pre- 

 served lenses ; the glabella is always expanded in front ; 

 the facial suture as in the last family ; pygidium with 

 entire margin. Dalmanites (Ord.-Sil.) has been described ; 

 Phacops (Sil.-Dev.) differs in the more expanded 



J 



a V, ^ b -- >C/" "-- c 



FIG. 61. HYPOSTOMES (LABRA) OF VARIOUS TRILOBITES. 



a, Elliptocephala asaphoides Emmons, Lower Cambrian. (X3-) (After 

 Walcott.) b, Ogygiocaris buchi (Brongniart), Llandeilian. (X.) 

 Dotted lines indicate outline of doublure. (After Salter. ) c, Asaphus 

 tyrannus Murchison, Llandeilian. ( X |.) (After Salter. ) d, Dalma- 

 nites caudatus (Briinnich), Salopian. (XJ.) (After Salter.) 



glabella, on which the segmental furrows are obsolete, 

 and the shorter, non-mucronate pygidium. 



General Geological History of the Trilobites. 



They appear suddenly in the Lower Cambrian, with 

 every appearance of having been long in existence 

 already, since specialized forms like Agnostus appear 

 alongside the primitive Mesonacidce. Most of the Cambrian 

 forms, however, are micropygous and unspecialized. In 

 North America different genera and species of Mesona- 

 cidce, serve as zone-fossils in the Lower Cambrian 

 (Nevadia, Elliptocephala, Callavia, Olenellus), and in 

 Britain and Scandinavia species of Payadoxides serve 



