Middle Devonian rocks in the Mackenzie River valley contain numerous smooth spiriferoid brachiopods of the sub-families Martiniinae and Ambocceliinae. Twelve of the fifteen recognized species first appear in significant numbers in the Middle Devonian Hume Formation. These twelve are representative of three genera, and five of the species and one genus have not hitherto been described. Certain species appear to be confined to the Upper Hare Indian and Ramparts Formations, and, as such, they may be useful stratigraphical indices.

The smooth spiriferoids are isochronously and heterochronously homoeomorphic with numerous, both closely and distantly related, brachiopod genera. Identification of the smooth spiriferoids is much complicated by homoeomorphy, and the phenomenon must be considered carefully in determining genera and species solely on superficial characters.

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Smooth spiriferoids in Middle Devonian time apparently thrived on a soft muddy bottom in waters of shallow depth. Most species probably were sedentary, but some may have lived attached to solid objects on the substratum by means of a peduncular holdfast, and others were at first sessile and later sedentary.

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