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 tessellatus, 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 cjeca 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. 



Harpides 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 casca are not correctly figured 

 by Barrande (Supplement, 1872, pi. I, fig. n), 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 

 c?eca without having any definite connection with them, but as a matter of. fact the radial 

 markings really diverge from the main trunks as in Elyx 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 



