THE EYES OF TRILOBITES 137 



(1) Holochroal (Fig. 97, A) : the visual area being covered with a con- 

 tinuous horny cuticle or cornea, which may be either smooth, so that 

 externally it gives no indication of its aggregate nature, or granular on 



m imp x gl. 



1 imD. ^\ 



,Uy. 



B 



Fig. 97. 



A — JEglinaprhca. Ordovician ; Vosek, Bohemia. Glabella (gl.) large, showing a 

 median impression (m. imp.), and paired lateral impressions (/. imp.) ; the fixed 

 cheeks are suppressed and the facet eyes (/. ey.), which are very large, occupy 

 nearly the whole area of the free cheeks. (After Barrande.) 



B — Harpes iingitla, showing paired ocelli (oc.) on the fixed cheeks (/. ch.). The free 

 cheeks are ventral in position and there are no facet-eyes. The cephalon, or 

 dorsal shield, has a wide, pitted, marginal expansion (m. exp.), which is pro- 

 longed backward on each side into a pointed spine. Ordovician, Bohemia. 

 (After Barrande, from von Zittel.) 



Fig. 98. — Phacops Latifrons. Bronn. Devonian. Gerolstein, Eifel 

 district. (From von Zittel.) 



The animal is rolled up and seen from the side. The prominent compound eyes 

 are of the schizochroal type. 



account of the unevenness produced by the facets beneath. According 



to von Zittel the lenses of the ommatidia are often visible by translucence. 



(2) Schizochroal (Fig. 98) : this type is limited to the single family 



