KONGL. 8V. VET. AKADEMIENS HANDLINGAR. BAND 34. \:<> 8. 29 



fcowards the interiör. They thus assume fehe asped of a composite coraJ with its septa in 

 fche calicles (Pl. \ r I fig. 31). This is also evidenl in Nileus palpebrosus and Dysplanus. 



2. Eyes with biconvex lenses. 



The surface of the eye is, as in Chirurus glaber \m... a mäss of contiguous hemi- 

 spherical lenses, probably once covered with a rnembrane, as is still to be seen in well pre- 

 served specimens of Bronteus laticauda. Both in Chirurus and Bronteus the lenses seen ver- 

 tically are globular and ordinated beside each other either continuous or separated only through 

 a faint dividing line. In a horizontal section passing righl through the poinl of contad 

 they show the common hexaedral shape and when somewhat corroded the interiör radiate 

 structure ;ilso comes forth, the radii directed towards a little black poinl in the centre. 

 The lenses of the Brontei have the same stellate structure as in Bumastus (Pl. II Gg. 

 7). In Cyrtometopus the lenses are in size the fourth of those in Chirurus and they 

 form an extremely thin stratum in strongest contrast with the adjoining cheek, which 

 surpasses them more than six tinies in thickness (Pl. III hg. 19). The lenses of Cyrto- 

 metopus are more flattened and irregular than in the former genera. The free cheek 

 around the eyes does not form a börder zone, somewhat imitating the eye structure as 

 in AsaphuSj but is more coinpact, composed of vertical elements which give to the tesl 

 of the trilobites in general a tendency to split up in vertical prisms. 



< )f a peculiar interest are the eyes in the oldest of all oculate trilobites, at pre- 

 sent with certainty known, Eurycare, Peltura, Sphaerophthalmus and Ctenopyge. 



Of these genera Eurycare is the oldest (see table p. 22). Amongst the many free 

 and detached cheeks only a single, very little one has been found with the eye ball fixed. 

 It seems to be of the same structure as in Sphaerophthalmus. In Sphaerophthalmus and 

 Ctenopyge the eye globes are enormous, considering the size of the cheek in which they 

 are set and occupy more than a third of the length of the free cheek (Pl. III f. 26, 31). 

 They are hemispheric, blackish and glossy, more so in the former genus. The spheroidal 

 lenses, projecting on the surface, are in Ctenopyge larger near the facial suture and small 

 at the opposite side where the eye is fixed in the free cheek. For the rest, in both ge- 

 nera (Pl. III fig. 34) the lenses form a thin stratum, where they in a vertical section lie 

 clongated, flattened and biconvex, slightly joined with each other at the point of contact. 

 The fine form which they exhibit reminds of the lenses of Sphaeroma. 1 They are in dia- 

 meter thrice as long as they are high. Seen in a horizontal section passing through the 

 point of contact they show hexaeders with a curiously jagged outline (Pl. III fig. 33). 



Peltura which is coeval with these, has a narrow semiglobose visual field (Pl. III figs 

 35 — 41), the superior surface of which is quite smooth and evenly rounded. On its in- 

 teriör side there stånd ont, somewhat distantiated, in a low relief semiglobular facets, quite 

 as regular incrassations of the cornea, thus not forming free lenses, but rather reminding 



1 BELLONCI Atti dei Lincei. Memorie, vol. X, 1881, Sphseroma, pl. II fig. 11. 

 K. Sv. Vet. Akad. Handl. Band 34. N:o 8 



