OLENELLUS AND OTHER GENERA OF MESONACID^. 2}f) 



Eye. — The changes in the eye lobes vary in different genera. All 

 agree in having a proportionally very long eye lobe in the youngest 

 stages, such as those represented by figs. 9 and 10, pi. 25, of EUipto- 

 cephala; figs. 20-22, pi. 25, of Padciimias, and figs. 5-8, pi. 31, of 

 IVanneria. The elongated eye lobes remain during life in most of 

 the species of all of the genera, excepting Wanneria. In this latter 

 genus the eye lobe is very long in the young [pi. 31, figs. 8, 7, 5, 

 and 4] and short in the adult [pi. 31, figs, i, 3 ; pi. 30, figs, i and 2]. 

 Two species of the genus Olenellus have short eye lobes : O. frc- 

 juoiiti [pi. 37] and O. canadensis [pi. 38]. The eye of O. fremonti 

 is unique in that it is larger in the adult [pi. 37, figs, i, 2, 3, and 6] 

 than in the young stages of growth [figs. 8-12]. This is one of the 

 characters that leads me to consider that the species is one that is 

 descendent from a species that had attained, as far as the eye was 

 concerned, the most advanced stage of development of any species 

 of the Mesonacidas, and then through reversion developed the long 

 eye lobe in the adult. This stage might be represented by the small- 

 eyed 0. canadensis. 



In one species I have been so fortunate as to find the outer faceted 

 surface preserved [pi. 43, figs. 5 and 6]. This surface is perforated 

 by minute rounded, hexagonal openings arranged in oblique trans- 

 verse rows which gives them a more or less quincunx order. The 

 interstitial spaces between the openings are narrow, rounded ridges. 

 There is no trace of a corneal covering, and the surface is so much 

 like that of the outer surface of the eye of Limiilns that I cannot 

 avoid the conclusion that they are of the same type [compare figs. 

 4 and 5, pi. 43], and "inward projections of the outer cuticle" 

 [Bernard, 1894, p. 422]. Bernard concludes that the eye of Limulns 

 is more primitive than- that of A pus. This may be a correct view, 

 but I strongly suspect that the primitive phylopods of the t3^pe of 

 Apus will be found to have developed earlier than the trilobites. 



Dr. A. S. Packard [1880, p. 508], after comparing the eye of 

 Linuilus with the sections of the eye of some Ordovician trilobites,. 

 notably Asaphus and Bathyurns, came to the conclusion that the 

 hard parts of the eye of the trilobites and of Liinnliis were through- 

 out identical, and that the trilobite's eye was organized on the same 

 plan as that of Limulns. Dr. G. Lindstrom, in his Researches on tlie 

 Visual Organs of the Trilobites [1901, pp. 26-27], found that there 

 were several types of eyes among the trilobites : 



