NATURAL CLASSIFICATION OF THE TRILOBITES 147 



preceding, characterized by having a rostral plate and by 

 the very tumid form of the large cephalon and the obscure 

 or obsolete boundaries of the glabella and occipital lobe. 

 The pygidium often closely resembles the cephalon in size 

 and form, and the axis is frequently scarcely defined. 



Considerable variation is shown in the size, position, and 

 direction of the visual surfaces. There is also a ratio be- 

 tween the size of the fixed-cheeks and the eyes. In propor- 

 tion as the fixed-cheeks are large, the eyes are small, and as 

 the area of the fixed-cheeks diminishes from a widening of 

 the axis of the animal, the eyes become larger. Thus, in 

 Rolocephalina, with extremely large fixed-cheeks and narrow 

 axis, the eyes are quite small. In Illcenopsis, Bysplanus, 

 Panderia, and Octilloenus they are progressively larger, and 

 in Illcenus, Bumastus, and Nileus, where the axis is wide and 

 the fixed-cheeks are reduced, the eyes are relatively large. 

 This variation reaches its limit in the species of jSi^glina, 

 where the axis is very wide and the fixed-cheeks are reduced 

 to almost nothing, so that the glabella and eyes make up the 

 entire dorsal surface of the cephalon. In JEglina princeps 

 Barrande the eyes extend about half the length of the cepha- 

 lon. The eyes of ^iJ. rediviva Barrande bound the whole 

 length of the sides of the head, and in ^-E. armata Barrande 

 the coalesced free-cheek pieces are almost wholly converted 

 into a visual area, so that there is a continuous eye around 

 the sides and front of the cephalon. 



Variations in the position of the eyes are to be noted in 

 nearly all the genera. In Ectillcenus and Psilocephalus they 

 are in front of the middle of the length of the cephalon, and 

 in Bygplanus, Illcenopsis, and Holoeephalina they are near the 

 posterior angles of the cranidium. Panderia has the eyes 

 directed obliquely backward, and in Thaleops they are carried 

 on conical extensions pointing outward. 



Family VII. ProStid^ Barrande. 



Cephalon about one-third of the whole animal; genal angles 

 generally produced into spines; glabella tumid, with two lateral 



