2. The primordial glabellar pleuron which was metamorphosed into a facial ridge is no visual organ. It is in the Olenidæ nothing but the elevated line made in the test by the subjacent main trunk of the circulatory system. It swells out in a node, »palpebral lobe», but not before the facial suture has been formed. In the genera where there is no facial suture, there is no node. In the Paradoxidæ where the ridge is of a different origin, there is no node, though there is a suture.
The four types of eyes in the trilobites have probably succeeded one another in the following chronological order:
1) with stemmata or ocelli; 2) biconvex or lentiform; 3) prismatic; 4) aggregate. The oldest known representatives for each type are for 1) Harpides rugosus in the Ceratopyge limestone of the Lower Silur., for 2) Eurycare, in the Cambrian Olenid schists, division 2, for 3) Megalaspis, in the Ceratopyge limestone of the Lower Silurian, for 4) Phacops in the Lower gray Orthoceratite limestone. The eyes of the trilobites show the greatest conformity with those of the recent Isopoda.
The most perfect eyes amongst all the trilobitic eyes may be those of the Phacopidæ, which are also geologically the youngest, the least developed again those of the Proetidæ or rather of the Bumasti. The great thickness of the cornea in these must have weakened their power of vision and they had probably only a faint perception of light.
On the maculæ of the hypostoma.
We shall now turn our attention to the visual organs which Liljevall discovered on the hypostoma of Bronteus. In doing this, we may bear in mind, that the genera in which we really have found lenses on the maculæ are relatively few, but that we shall review the maculæ in all genera, which we have been able to examine, and try to show that even most of these may, although in an inferior degree, have acted as visual organs. At the same time some more details shall be given about the cephalic eyes for comparing them with the hypostomic ones.
Acidaspis Murch.
The hypostoma is of a peculiar type, deviating from that of the other genera, squarish or rectangularly transverse, entirely without terrace lines and no maculæ proper. Barrande has given no less than nine different samples of these hypostomas.
Acidaspis crenata Emmr.