Article: Species of Tretaspis (Trilobita) from the Ashgill Series in Wales
Nine species of Tretaspis are recognized from various horizons of the Ashgill Series in Wales. Tretaspis colliquia Ingham is regarded as a sub-species of T. moeldenensis Cave. Four populations of Tretaspis are placed in T. moeldenensis (s.l.) and considered to occupy intermediate positions within the plexus connecting T. moeldenensis colliquia and T. m. moeldenensis; for the latter, large topotype samples also give increased knowledge of the fringe characters. T. cf. radialis Lament occurs throughout the bulk of the Sholeshook Limestone Formation (south-west Dyfed) but is replaced in the topmost part by T, aff. radialis. The pygidium of an indeterminate species from the Birdshill Limestone at Llandeilo resembles that of T. m. moeldenensis. T. hadelandica brachystichus Ingham is described from the highest Sholeshook Limestone and succeeding Slade and Redhill Mudstone Formation, T. cf. latilimbus distichus Ingham from the high Ashgill of the south-west Berwyns and T. cf. sortita (Reed) from the topmost Ashgill of the Meifod area. An indeterminate form from the Slade and Redhill Mudstones and T. cf. calcaria Dean from the Rhiwlas Limestone are not yet placed within either of Ingham's two main species groups.