Genus Trigonodictya Ulrich, 1893

Trigonodictya Ulrich, 1893: 160 .

Astreptodictya Karklins, 1969: 49 .

Trigonodictya (Astreptodictya) – Karklins 1983b: 513.

TYPE SPECIES. — Pachydictya conciliatrix Ulrich, 1886 by subsequent designation (Ulrich 1893). Decorah Shale, Middle Ordovician; United States (Minnesota).

DIAGNOSIS. — Branched colonies, sometimes with lateral ridgelike expansions. Mesotheca straight to sinuous in longitudinal section, locally zigzag in transverse section, containing median rods. Autozooecia arranged in straight rows, subrectangular to subrhomboidal in transversal section of endozone, locally separated by extrazooecial vesicles in endozone, separated by extrazooecial stereom in exozone, rectangular in deep tangential section, becoming oval on the colony surface. Basal diaphragms straight to slightly curved. Extrazooecial skeletal deposits common, consisting of laminar and vesicular portions. Vesicular structures common in inner exozones, locally in endozones. Laminar stereom commonly with dark zones, longitudinally aligned, locally with indistinct mural styles. Autozooecial boundaries distinct, delineated laterally by continuous dark zones. Monticules absent.

OCCURRENCE. — Middle Ordovician – Middle Silurian; Europe, North and South America.

COMPARISON

Trigonodictya Ulrich, 1893 differs from Pachydictya Ulrich, 1882 in regular arrangement of autozooecia in straight rows.