30 LINDSTRÖM, VISUAL ORGANS OF THE TRILOBITES. 



of the for the rest differently formed quasi-lenses of Limulus. In a vertical section they 

 appear as the interiör raoiety of real ovate lenses (Pl. III f. 40 — 41). 



The much younger Acerocare has a similar cornea. A very little specimen, the 

 head of scarcely more than one millim. in length, retains both eyes, of which one shows 

 the slightly convex lenses and the other a cast of the interiör side as in Feltura. These 

 both genera should in consequence of their peculiar limuloid cornea be ranged for them- 

 selves apart from the real lenticulate genera, but any material sufticient for doing this 

 properly, is at present not at hand. 



II. Aggregate eyes. 



These are found solely in the family of the Phacopidee, unless the Lichadida* were 

 a Ko provided a\ ith this sort of eyes, hut we ha ve had no opportunity to study them. 

 It seems, however, not likely that they had aggregate eyes. Barrande has represented them 

 quite as finely reticulate as the eyes of any Asaphid. We have sectioned and figured the 

 eyes of Dalmanites vulgaris and D. obtusus from the Silurian of Gotland and found that 

 these have truly aggregate eyes, each consisting of a regular biconvex lens, lying 

 enclosed in a socket of its own and covered by a cornea of its own. The distance between 

 the eyes is much variable and in a few instances they are nearly contiguous. Extremes are 

 seen on pl. III figures 43, 47. The lenses are comparatively large, and have always had 

 a covering membrane, though this in many instances has been löst. This membrane which 

 is an immediate continuation of the general integument of the body covers the lenses all 

 round their superior moiety. In its prolongation downwards between the lenses (Pl. VI 

 tig. 3, 4) it is free from the contact with them and hängs alongside and around 

 much incrassated, so as to take in a section a lengthened lancet like shape. It lies 

 thus alongside the other interstitial test. and is like this perforated by longitudinal ca- 

 n.ils. In a horizontal section taken a little below the surface it encircles the lens as a 

 wall like ring (Pl. VI tig. 1, 2). In a vertical section the lenses lie in direct contact 

 with the cheek without any inter\ ening zone and the cheek has the structure so common 

 amongst the trilobites, being perforated by vertical tubes going straight down from the 

 surface (Pl. III hg. 44, Pl. VI f. 5). 



In Dalin, vulgaris and also in Phacops quadrilineata there is as already before men- 

 tioned a peculiar structure beneath the lenses, consisting of narrow, threadlike, straight 

 lines, twice as long as the lenses (Pl. III 49 — 50, pl. V tig. 38). In a horizontal section 

 they are found to be irregular prisnis closely packed. It can not be any structure pecu- 

 liar to the eyes or the lenses, ratlier some parasitic growth added sinee the death of the 

 animal. The lenses are in several specimens composed of clear calcareous spar. In others 

 again they have been filled with a dark muddy calcareous rock excepting in the lower 

 moiety where there is left a residue of the white spar, having in all lenses assumed a 

 regular shape which I consider as organic (Pl. VI tig. 5). This spar covers the whole 

 bottom and its upper rim is incrassated and bent inwards. In horizontal sections 



