Pt. angustifrons Dalm.
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.
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.
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.