New Ordovician cornutes (Echinodermata, Stylophora) from Montagne Noire and Brittany (France) and a revision of the order Cornuta Jaekel 1901

Plate homologies are identified and discussed in cornute stylophoran echino derms. The main results are: 1) the homology of the posterior zygal plate i n all cornutes and, 2) the non-homology of the spinal process, which can be borne by two distinct plates from the marginal frame. A functional analysi s of stylophoran "accessory orifices" as exchange systems is realised: they are interpreted as respiratory structures. Sutural pores of Phyllocystis b layaci and cothurnopores could represent exothecal pore-structures, and lam ellipores endothecal. pore-structures. Other possible means of respiration are also envisaged in cornutes. A systematic revision of the Order Cornuta is also presented. The new genus Arauricystis is proposed for two species o f cornutes previously assigned to the genus Cothurnocystis. Two new species of cornutes from the Lower Arenig (Lower Ordovician) of Montagne Noire (So uthern France) are described, Ampelocarpus landeyranensis nov. gen. et nov. sp. and Thoralicystis ubaghsi nov. sp.. A new cornute from the Llandeilo ( Middle Ordovician) of Brittany (Western France), Scotiaecystis guilloui nov . sp. is also described. Finally, a cladistic analysis of cornutes confirms the results obtained by the identification of plate homologies: 1) cornute s and mitrates are sister-groups, 2) Ceratocystis belongs to the stem-group of both cornutes and mitrates, 3) Amygdalothecida and Cothurnocystida are sister-groups and, 4) Protocystites belongs to the stem-group of both Amygd alothecida and Cothurnocystida.