THE EYES OF TRILOBITES 139 



The possibility of there being more than one pair of median eyes — 

 or even of lateral eyes — in trilobites is indicated by the distinct evidence 

 of metamerism in the glabella and other parts of the head. There are 



1m ttl. Q 



Fig. 100. — Median Eye Tubercle in Tretaspis Seticornis (His). 



A, ■ 80 ; B, 36 ; C, D, 50. A, C, D showing five small pits on the tubercle. 

 A-B : meraspid stage II. C-D : adult. B : lateral view of cephalon with 

 the big median tubercle, m.e. A-B : from the Trinucleus shale Dalarne. 

 C-D : from the upper Trinucleus limestone, jr. : pseudo-frontal lobe. 

 Frogno Ringerike. (After L. Stormer.) 



Fig. ioi. — Transverse Section through the Median Eye Tubercle (m.e.) of 

 an Adult Specimen of Tretaspis Kia^ri n.sp. (After L. Stormer.) 



marked variations in the size and form of the glabella in different species 

 of Trilobite. In the typical forms, however, it is seen to be composed of 

 five segments (Figs. 103 and 104); these are separated by four transverse 

 or oblique grooves. The anterior segment is known as the frontal lobe 

 and sometimes bears a single median impression, or there may be three 

 pits, one median usually in front and two lateral. The posterior segment 



