Part 16

The maculæ, seen in a thin section from the inside, are reticulate, fig. 49, or of a pattern exactly like that in the changed eyes or maculæ of Illænus and even Asaphus, and thus indicating that it was composed of short prismatic lenses. The vertical section of a macula of a specimen from another locality rather suggests the presence of large sphærical lenses than elongated prisms. The great thinness of the macula in contrast to the excessive thickness of the shell around the eye is seldom so evident as here.

A specimen of an unknown species, found detached, has the macula? unlike the other species, oblique with the inferior end directed inwards, placed on the top of a sort of a little elevation, fig. 47.

=Megalaspides= BRÖGGER.

This author has a figure of M. dalecarlicus in II pl. 1 fig. 19 and also of an unnamed species fig. 20, both showing globular maculæ. In HOLM'S original description of his Megalaspis dalecarlicus (Trilobiten des Phyllograptusschiefers Dalecarliens figs. 8, 9) the same hypostoma is also figured. This genus seems to have a closer affinity with Ptychopyge than with Megalaspis.

=Megalaspis= ANG.

The following figures of hypostomas with maculæ have been published.

Megal. limbata by BRÖGGER I, tab. XII, fig. 10, and in II pl. 2, fig. 22, somewhat differing from the first figure.

Meg. planilimbata BRÖGGER II tab. 2 figs. 21, 21 a. Both figures incomplete and incorrect.

The hypostoma has a large central ovate gibbosity with the maculæ near its basis, surrounded by the winglike expanded limbus.

=Megal. attenuata= WAHLENBERG.

Pl. V figs. 1-6.

The long and narrow maculæ are crescent-shaped and project beyond the hypostoma supported on the superior surface of a sharp extenuated edge where they rest as on a shelf. They consist, as seen in vertical sections, figs. 5, 6, of alternating rows of light and black streaks, and in a horizontal section the spongious network appears, as it is found in Asaphus and others and thus indicating the tendency of this organ to assume a prismatic structure though undeveloped.

The cephalic eyes in M. attenuata form the most regular hexaedral prisms fig. 1 a, short and covered by a thin cornea. The border zone of the eyes has a structure exactly like that of the maculæ, the same as in the spongious zone of Asaphus.

=Megal. limbata= BOECK.

Pl. V fig. 7.

We give a new figure of BRÖGGER'S original specimens, as his figure 23, Tab. II is incomplete and incorrect. The maculæ are situated a little higher up on the sides of the central gibbosity.

=Megal. planilimbata= ANG.

Pl. V fig. 8.

The maculæ are crescent-shaped and not so prominent as in Meg. attenuata and the visual spot in the same position as there.

To observe is that the general structure of the hypostomic shell in this and many other species is prismatic.

=Nileus= DALMAN.

Hypostoma with maculæ figured in N. armadillo by ANGELIN pl. XVI fig. 5 c, and BR. II pl. 3 fig. 40.

The hypostoma in this genus is transversal with large lateral expansions, the central convexity only faintly elevated and almost coherent with the posterior part. The maculæ lie exactly on the horizontal median line of the hypostoma. The anterior wings are well developed like slightly bent acuminated horns and the posterior wings are conical points. The terrace lines are transverse and a little wavy.

=Nileus armadillo= DALM.

Pl. V figs. 10-15.

There is some variability in the hypostoma of specimens from different localities. The most known and numerous specimens are from the renowned localities of Östergötland. They are decidedly transverse, the proportion of the hight to breadth is as 3 to 5 and the narrow border runs all round. In specimens from Kongslena, Vestergötland, the proportion is as 15 to 22, in specimens from Sandvik, Öland, again, we have 13 to 23 or 6 to 11. The maculæ, fig. 14, are oblong, inwardly pointed, quite smooth, with a little concavity in their centre, and their surface is slanting obliquely towards the surface of the hypostoma, almost as in Niobe. There is no trace of any structure.

The cephalic eyes are covered by a comparatively thick membrane, a direct continuation of the general test, fig. 12, hiding a row of prismatic lenses. In some specimens elongate, clear, crystalline rods continue downwards from the bases of the lenses and are probably nothing but inorganic crystals of calcareous spar, fig. 11.--The eyes are larger comparatively than in other species.

=Nileus (Symphysurus) palpebrosus= DALMAN.