Part 20

=Ptychopyge= ANG.

Previous figures of hypostomas with maculæ are the following:

Pt. aciculata BR. II, pl. 1 fig. 13.

Pt. glabrata BR. II, pl. 1 fig. 14.

=Pt. aciculata= ANG.

Pl. VI figs. 40, 41

The broad hypostoma bears the elongated, acuminated maculæ in an oblique direction, contrary to what is seen in most other species. The anterior end slopes inwards and the lower end outwards, the surface, which in the cast drawn is surrounded by a marginal line, is directed sidewards against the broad lateral wings and is obscured by the declivity formed by the groove. The same disposition of the mature also occurs in Ptych. glabrata. These two species consequently in this respect differ essentially from the other four species which we have examined, but there is for the rest no reason to make these form different genera or subgenera.

=Pt. angustifrons= DALM.

Pl. VI figs. 31-36.

There are two varieties of hypostomas: one broad fig. 32 and one more elongated fig. 33. The maculæ having a convex surface lie at the inferior end of the lateral grooves prominent on a semicylindrical support, slightly inclining outwards. In the broad variety they are oblique (fig. 34) and in the elongated variety they are horizontal (fig. 35). In a vertical section fig. 36 we have the same indications of a prismatic arrangement in the maculæ as in the Asaphi. The horizontal section does not represent so clear and distinct a reticulate structure as in the Asaphi, the prisms are much smaller.

The cephalic eyes are composed of closely packed hexaedral prisms, the gradual change from solid homogenous ones to those which are interiorly destroyed, only with the exterior form preserved is easily seen. The figure of a horizontal section (fig. 31) gives a good idea of the peculiar destruction of the interior of these prisms. In the vertical section the prisms are not discernible.

=Pt. cincta= BRÖGGER? according to FRIEDR. SCHMIDT in Mus. Holm.

Pl. VI figs. 37-39.

The hypostoma which BRÖGGER figures (II, pl. I fig. 2 a) as belonging to this species is completely identical with that of Pt. angustifrons, while the specimens which FR. SCHMIDT has marked out as being the true Pt. cincta have a different hypostoma. The maculæ vary in the specimens, being in some placed horizontally or in a right angle to the longitudinal axis of the hypostoma, in others, again, a little obliquely outwards. They form prominent, free lying tubercles. It is from the superior face that the rudimentary lenses have left their traces, as seen in a longitudinal section (fig. 39). It is consequently this superior surface which is the macula proper.

=Pt. glabrata= ANG.

Pl. VI fig. 42.

We have given a figure of the elliptic acuminate maculæ as seen on a cast of the inside of a hypostoma, the same which is the original to fig. 14 in BR. II.

=Schmidtia= MARCOU.

MOBERG has in his paper on »Sveriges äldsta Trilobiter», pl. 15, fig. 6, drawn a hypostoma of Schmidtia Torelli with scars of maculæ, placed like that of the Paradoxidæ. There is no mention made of them in the descriptions.

=Trochurus= BEYRICH.

=Tr. pusillus= ANG.

Pl. IV figs. 52, 53.

The strange transverse, bipartite hypostoma carries the small, round, tuberculose maculæ on the superior moiety. As seen from the interior (fig. 53) they are obscurely maculate, but it is doubtful whether these black dots are derived from lenses, as the surrounding hypostomic surface also has the same mottled aspect.

=Trochurus= sp.

Pl. IV figs. 54, 55.

This hypostoma belongs to an unknown species of Trochurus from the Southern Gotland (Burs). Its macula are placed almost as in the preceding species, but are much larger. They are surrounded by prominent margins. No lenses have been discovered on them.

CONCLUSIONS.

In reviewing the genera and species enumerated above, as to their number in which the structure of the macula has been observed, we arrive at the following results.