[30] Bullar Philos. Transact. 1878, pt. II, pl. 46, fig. 12.
[31] Bellonci Atti dei Lincei, Memorie X 1881, pl. II, fig. 11.
[32] See Grenacher pl. II f. 18 Epeira.
In Phacops quadrilineata the lenses are more elliptic than in the former. On their interior surface beneath the spiny tufts mentioned large hexaedral prisms of clear calcareous spar issue, one prism for each lens ([Pl. V fig. 38]), having thus a very deceptive appearance, but no doubt of inorganic origin with the lenses as a basis for their crystallization, quite as in the Cystoids where the interior often is converted into a mass of crystalline prisms, issuing from the interior surface of the plates.
III. Genera with stemmata and ocelli.
In a little group that has retained larval or ancestral characters during a great part of the palæozoic period, the genus Harpes stands as a type. It has ranged from the oldest Lower Silurian, if we join the related Harpides, to the middle Devonian. From near the top of the glabella, though not so much forward as the facial ridge of the blind trilobites (Olenus, Liostracus etc.) a straight ridge of much varying length stands out on both sides and at its extremity two or three globular stemmata with glossy surface lie encased. Probably this ridge has the same origin as in the Olenidæ, the more so, as there are indications of an extensive circulatory system. On [plate IV fig. 18-19] the right hand ocelli of Harpes vittatus Barr. from Lochkow, Bohemia, are represented. They are two, lying isolated near each other, quite globular with circular outline, smooth and glossy as to exhibit a shining surface. Their size is 0,4 mm in diameter. Being cut vertically in the direction of the longitudinal axis of the head they resemble elongated hemispheres, convex on the exterior surface, slightly concave on the interior. The test of the head lies between them as a saddle and covers them only partially and on the outer sides they lie with their margins encased in the head shield. Seen in thin sections of the right lens and magnified the whitish mass is traversed by vertical, blacker streaks, standing somewhat radiating towards the sides and cancellate. The other lens has a horizontal row of black dots. All this is probably not of any structural value, only due to later changes.
The remarkable genus Harpides from the lowest Lower Silurian belongs also to this group and has beside the fixed peduncle a peculiar elongated ridge going from the eyes to the lateral margins of the head[33], a ridge which is also present in some of the true Harpes.
[33] Harpides breviceps cannot belong to this genus and is rather related to Erinnys Salter, as also Matthew holds it.
As Beecher has shown[34] the larva of Trinucleus possesses quite the same transverse ridge with intumescent eyelike extremities, and although the smallness of the specimens has not permitted to ascertain the presence of a true eye, it may be apposite to suppose it on the homology with the eyes of Harpes, a genus with which Trinucleus is related. But as well known, in the adult Trinuclei there is no trace of these ocular ridges nor of real eyes so that Beecher in his paper »Blind Trilobites»[35] numerates Trinucleus amongst these. Out of the nine Scandinavian species of Trinucleus no less than seven have a well marked little tubercle on each side of the glabella placed exactly on the same spot where the larval Trinuclei had their much larger eyes placed. In this case it may be allowed to suppose that the tubercles are the direct successors of the larval eye and that they are true ocelli. Reede seems to be willing to regard them as possessing a visual function.[36] Beecher[37] holds the eye nodules in the larva and the ocelli in the adult to be identical.
[34] Structure and appendages of Trinucleus.