84 THE APPENDAGES, ANATOMY, AND RELATIONS OF TRILOBITES. 
of the compound eye, and the eye-line here the homologue of the eye-line in Ptychoparia, 
why does it continue beyond the eye? In any case, it can not be interpreted as a nerve. 
Cryptolithus tcssellatus, when the cephalon is 0.45 mm. to 0.65 mm. long, shows short eye- 
lines and a small simple eye on each cheek. In some half-grown specimens, traces of the ocelli 
can be seen, but the eye-lines are absent. In the adult, both the eye-lines and the ocelli are 
entirely wanting. Reed states that "nervures" are also absent, and so they are from most 
specimens, but well preserved casts of the interior from the Upper Trenton opposite Cincin- 
nati show them, and one cheek is here figured (fig. 25). As apparent from the figure, the 
main trunk is very short and gives rise to two principal branches, the first of which in 
its turn sends off lines from the anterior side. It was a specimen showing these lines which 
Ruedemann (1916, p. 147) figured as showing facial sutures. The interest lies in the fact 
that while the ocelli and eye-lines were lost in. development, the genal c?eca are present 
in the adult, showing that they are different structures. 
Fig. 25. — Cryptolithus 
tessellatus Green. Side 
view of the cheek of a 
specimen from the top of 
the Trenton opposite Cin- 
cinnati, Ohio, to show the 
branching genal caeca. 
These are the "facial 
sutures" of Ruedemann. 
Harpidcs is another genus in which genal caeca are strikingly shown, and in this case 
they completely cover the huge cheeks, radiating from two main trunks to the front and 
sides. I have seen no good specimens, but it would appear from Angelin's figure (1854, 
pi. 41, fig. 7) that the rather large, simple eyes are not situated exactly on the vascular trunks. 
In the Harpides from Bohemia, the main trunks extend out with many branches beyond the 
simple eyes. It should be stated that the courses of the genal creca are not correctly figured 
by Barrande (Supplement, 1872, pi. 1, fig. 11), as shown by casts of the original specimen 
in the Museum of Comparative Zoology. From Barrande' s figure, one would suppose that 
the eye-lines and their continuation beyond the "ocelli" were superimposed upon the genal 
caeca without having any definite connection with them, but as a matter of fact the radial 
markings really diverge from the main trunks as in Elys and similar forms. 
Summary. 
As Reed has said, these lines are not mere ornamentation, but rather represent traces 
of structures of some functional importance. They probably can not be explained as traces 
of nerves and more likely represent either traces of the gastric caeca or of the circulatory 
