STRUCTURE AND APPENDAGES OF TRINUCLEUS 223 



not been demonstrated, it has generally been called an ocellus. 

 It is more clearly preserved in adult specimens, though it can 

 be detected in young examples, as indicated in figures 1, 2, 

 Plate X. 



An eye-line occurs in many early trilobite genera, and is 

 well marked in Conocoryphe, Olenus, Ptychoparia, and Arethu- 

 sina. At least four-fifths of the Cambrian forms preserve 

 this feature, which is almost entirely eliminated before Devo- 

 nian time. It differs in extent, but not necessarily in nature, 

 from the eye-line of Trinucleus and Harpes in running 

 entirely across the fixed-cheeks to the free-cheeks, ending in 

 the palpebral lobe in eyed forms. It is evidently a larval 

 character in the trilobites, as shown from its geological 

 history and the ontogeny of Trinucleus. From the direction 

 of the optic nerve in Limulus^ and its relations to the surface 

 features of the cephalothorax, the eye-line probably repre- 

 sents the course of that nerve, and is of much less morpho- 

 logical importance than the different types and arrangement of 

 visual organs. 



The pygidium of young T. concentricus (Plate X, figure 3) 

 is remarkable for the lack of definition between the axis and 

 pleura. In later and adult stages the number of ridges on the 

 pleura and axis do not correspond, and from figures 4, 5, and 

 6 it is evident that in this genus the number of pleura is 

 no indication of the number of pygidial segments or pairs of 

 appendages, which, however, may be shown, as in this case, 

 by the annulations of the axis. In this respect the pygidia 

 in Encrinurus, Cybele, and Dindymene are of the same nature. 

 Figure 6 also shows a narrow, striated doublure, a character 

 generally overlooked in descriptions of Trinucleus. 



Appendages. 



Three specimens have been thus far observed which show 

 the nature of the appendages in Trinucleus. Two of these 

 are illustrated in figures 4, 5, and 6 of Plate X. Figure 4 

 represents the thorax and pygidium viewed from the dorsal 



