28 LINDSTRÖM, VISUAL ORGANS OF THE TRILOBITKS. 



The lenses, as seen in a vertical section of the eye of Asaphus expansus, (pl. I 

 fig. 12), are columnar prisras, like the pillars of basalt, attaining a length of 0,2 rara 

 and at the point where the eye joins the test of 0,3 rara. At their interiör extreraity 

 they have a breadth of 0,066 min. On that point the surface is convex and at the ex- 

 teriör surface plane. They are closety packed and in a transverse seetion resemble a 

 pavement of regular hexaeders. But they also assume other shapes and become rhorabs 

 or even quadrates, as seen in a specinien of Asaphus fallax (pl. I tig. 18), where the 

 hexaeders and quadrates lie side by side without transitional forms. As a rule the lenses 

 become more and raore irregular in the vicinity of the surrounding fratne or near the 

 suture, nearly blotted out, as it were, and without any definite börder line mingled with 

 the confused, spongy mäss that like a belt or a frame surrounds they eye in Asaphus 

 and is sharply limited from the other part of the free cheek. This remarkable zone 

 which is almost only present amongst the Asaphidae (Asaphus, Megalaspis, Ptychopyge, 

 Isotelus) retains in a confused manner soraewhat of the prismatic structure of the eye 

 as shown in the section (Pl. I, fig. 11, b). The eye of Bumastus also is environed by 

 a similar zone, with a structure like that of the eye (PL II fig 35, 41), 



In an undetermined species of Asaphus the lenses, although soraewhat apart, are 

 of an elongated hexaedrie outline, which passes into a regular circular one farther away 

 ;iii<1 on the surface of the eye they are slightly convex (Pl. I tigs 27 — 29). In other 

 genera belonging to this group the shape of the lenses are like those of Asaphus, so for 

 instance in Illsenus (I. chiron and I. Esraarcki) and in Niobe. In Dysplanus centrotus 

 they are shorter and broad, and their interiör or lower surface strongly convex. It is 

 likewise so in Nileus, where Nilens armadillo has an exceedingly thick exteriör integu- 

 nient above the lenses. Such an integument has in a still higher degree increased in 

 Bumastus sulcatus so as to exceed in thickness the stratum of the corneal prisras and 

 it raay in fäet be doubted if the eyes of this species ever were able to function as visual 

 organs. Proetus nearly reserables Bumastus in the thickness of the integument covering 

 the prismatic lenses, which are interiorly convex, with a diameter of 0,03 mm. 



In all genera belonging to this group a horizontal section gives the image of the 

 hexaeders as in Asaphus with some change to squares or rhorabs. 



In scrutinizing a horizontal, soraewhat extensive section of an eye in this group of 

 trilobites, it will be perceived, as for instance in the figures (Pl. V fig. 16, 22) that the 

 regular and evidently homogenous and intact prismatic lenses by and by have been al- 

 tered and in a, part of the section, a little distant from the intact ones disintegrated in 

 their interiör, showing various aspects of alteration. I cannot but think that this is a 

 destruction which has set in long after the fossilization. It has revealed certain states 

 of the intiraate structure, certain delicate details, that now with an astonishing regularity 

 come in sight and probably also lie hidden in the intact prisras. In the specimens of 

 Asaphus, which we have studied, the alteration has taken the shape of a concentric stra- 

 tification forniing the body of the prisras, which is well discernible in a horizontal sec- 

 tion, but not easy to detect in the longitudinal one (Pl. I figs 9 — 10, 11). It is likeAvise 

 so in Niobe. In the other genera again the decoraposition makes the prisras look like erapty 

 tubes in which a few irregular traverses and trabecular remains of their solid mäss radiate 



