During my study of Norwegian Trinucleids, a number of microscopic sections were made. In some cases, the sections gave good conditions for examining the shell structures. The shell structure was described in details in one species, Tretaspis seticornis (His), (Størmer 1930, p. 96) . The specimens were preserved under favourable conditions.
With the hope of finding further details of the shell structure in trilobites, several slides have been made of different trilobites from different horizons and localities.
Masses of trilobite fragments are not very frequent in the Ordovician layers. Usually one or two specimens only, are present in one section.
Sections have provisionally been made of: Asaplws expansus (Linn.) and Illænus dalmanni Volb. from the Expansus shale near Oslo; Ogygiocaris dilatata Brünn. from the Ogia shale near Oslo, Remopleurides sp. and Trinucleus sp. from the Ampyx limestone at Ringerike; Trinucleids from the upper Chasmops limestone, lower Trinucleus limestone, Trinucleus shale and upper Trinucleus limestone at Ringerike; from Trinucleus Iimestone and upper Chasmops zone near Oslo. The lower Trinucleus limestone from Ringerike also contains large numbers of Ampyx sp.
Together with the Norwegian species, some Baltic ones of Asaphus have been examined in sections.
Of the material mentioned, only a few species show details of the original shell structure. In Tretaspis seticornis (His) from the upper Trinucleus limestone, Ringerike, we find an outer dark layer, the "pigment" layer, two pellucid layers, the "principle" and "inner" layer. The layers are pierced by very minute tubulae running perpendicularly to the surface. Distinct laminae are visible in some cases. In addition to the fine tubulae mentioned, bigger canals are present in certain definite parts of the shell. These are only found in a section of Trinucleus bronni (Boeck) (p. 101) .
During the study of these structures some forms were discovered which could not be regarded as a part of the regular shell structures. Some club-shaped cavities in the outer part of the shell must be regarded as being traces of foreign organisms. These structures will be described in this paper.