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.
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.