A silurian gastropod fauna from the Arisaig group of Nova Scotia.

A fauna of more than sixty species of Silurian Gastropoda is descrihed from the Arisaig Group of Nova Scotia. Two new genera, Anapetopsis and Arjamannia, are proposed and thirteen new species: Pharetrolites bambachi, Plectonotus boucoti, Plectonotus cherylae, Tritonophon kivitalonae, Anapetopsis maclearni, Anapetopsis lalajae Mimospira abhae, Arjamannia aulangonensis, Haplospira sibeliuxeni, Naticonema kauffmani, Cyclonema (C.) lydiamariarum, Naticopsis trevorpatriciorum and Morania (?) boltoni. Particular emphasis is given to the systeaiatics, ontogeny, functional morphology and palaeoecology of members of the subfamily Plectonotinae (Bellerophontacea) which characterise much of the Arisaig Group. The palaeoecology of other gastropod associations from the Arisaig Group is discussed and the sequence of faunas is related to the advance of barrier conditions into the shallow marine environment prevailing at Arisaig during the Silurian.