58 LINDSTRÖM, VISUAL ORGANS OF THE TRILOBITES. 



hitherto have been delineated. This is Illaenus angustifrons var. depressa Holm I, pl. VIII 

 fig. 18. A little, fragmentary hypostoma with globular maculae. 



Illaenus Ghiron Holm. 



Pl. IV figs. 22—25. 



Tlie hypostoma has the shape characteristic of all true fflaeni, viz. a straight anterior 

 margin with two large flat rectangular ov nearly quadratic wings, a globular or sphserical 

 median field, forming the chief portion of the exteriör surface, which is covered by some 

 rare, tine, terrace lines. In some other Illseni the surface is smooth. At the posterior 

 base of the median field the two maculae are placed. tinv, oblong, but lying on the same 

 level, at right angles to the median longitudinal axis of the hypostoma. They are covered 

 with lenses of irregular and indistinct shape fig. 25. I do not think that they in this 

 and the other species of Ilkenns are lenses of the same conformation as in Bronteus, 

 they are rather the tops of the subjacent lenses which are of the prismatic form. The 

 cephalic eyes do not show any lenses on the outside. They have a narrow, opaque ex- 

 teriör integument, hiding hexaedral, thickly packed, straight prisms, forming a stratum 

 in thickness of 0,2 millim. 



The hypostoma consists of several thin layers of superposed calcite, which easily 

 peal off, so that it is seldom that the true exteriör seulptured surface remains. 



IHii-mis Esmarcki Schloth. 

 Pl. IV tigs. 2 G— 33. 



This the most common of all the I Hörni has in several specimens shown the hypo- 

 stoma with the maculae in situ. The hypostoma is smooth excepting on the marginal 

 ridges where there are some terrace lines. On the median axis of the central gibbosity 

 (fig. 30) tive faintly visible transverse folds are situated, a curious feature which we have 

 not observed in any other trilobite. The maculae are exceedingly small, fig. 31, elongate, 

 spindleshaped and accuminated in both ends. On their surface there are accumulated 

 several lenslike globules. As the horizontal sections show there are, however, no regular 

 lenses, but a network of polygonal meshes with radii in the hollows, fig. 32. The perfect 

 identity of this structure with that of certain States of the cephalic eyes will at once be 

 perceived by comparing this figure with figure 26 representing the same sort of polvedric 

 meshes, though a little more regular in the cephalic eve, where, however, the irregularity 

 also prevails near the periphery of the eye. If we now compare the sections of the 

 cephalic eye of fllaenus Chiron and the section of the regular and nearly unchanged eyes 

 of 111. Esmarki with the second section of the latter, we cannot avoid the conclusion that 

 we in those of 111. Chiron, figs. 22, 23, and the first of 111. Esmarki, fig. 27, see the 

 primitive and intact state of the lenses and in the last sections of 111. Esmarki, figs. 26, 

 28, the changed and deteriorated state of the same prismatic lenses. 



The vertical section of the hvpostomic cvcs fig. 33 shows in their present changed 

 condition an evident longitudinal direction of the chief elements, quite as the lenses of 





