The transgressive sequence of the Ordovician intracratonic basin of the present-day Lac-St-Jean and Chicoutimi areas is characterized by a variety of sediments that were deposited in a number of environments. Marginal and nearshore sediments of the basin were a complicated system of clastics derived from the adjacent rugged coastal hinterland and intertidal and shallow subtidal carbonates with variably admixed clastics. Farther from shore, in sheltered bays or lagoons partly enclosed by offshore shoals and bars of skeletal lime sands, fine-grained carbonates with coral-algal-bryozoan thickets accumulated. These sediments exhibit numerous submarine hardgrounds or firm grounds. Variations in sedimentation rate and the development of periodically exposed banks of sediment also resulted in the formation of subaerial microkarstic surfaces. The offshore skeletal shoals and bars, which were composed mainly of pelmatozoan debris but included patches of incipient reef growth, provided an incomplete barrier to circulation. Deeper water offshore subtidal sediments consisted of mixed carbonate and argillaceous muds.Sudden increased rates of transgression, probably related to isostatic movements, resulted in the drowning of these coastal environments and the deposition of dark-coloured argillaceous shales in deeper water. These shales are partly of latest Utica and mainly Lorraine in age and it is likely that the initial submergence of this part of the Canadian Shield was much later than that in the St. Lawrence Lowland, probably occurring in Cobourg time.

The Middle Ordovician reefs of Norway were the first to develop in the western part of the Balto‐scandian epicontinental sea and are the earliest coral‐stromatoporoid reefs so far reported in Europe. Small patch reefs in the Steinvika Limestone, Langesund‐Skien district, consist mainly of algae, echinoderms, corals and stromatoporoids. Bryozoans, molluscs, arthropods and brachiopods are also present. The reefs developed on pelmatozoan‐rich substrates and are organically zoned, consisting of a pioneer community of stemmed echinoderms and sheet algae, a high‐diversity intermediate community dominated by fasciculate corals and a low diversity climax community of massive corals and stromatoporoids. These communities are interpreted as the seral stages of an autogenic ecological succession. Small patch reefs are also present in the laterally equivalent Mjøsa Limestone, Toten and Nes‐Hamar districts. These are organically very similar to those in the Steinvika Limestone and developed in an identical way. A large complex, consisting of several reefs, is also present in the Mjøsa Limestone. Unlike the reefs elsewhere, which developed within shallow inshore areas, this complex developed at the outer edge of the inshore shelf. The outstanding feature of the complex is the main reef forming the offshore limit which is totally dominated by stromatoporoids and lacks a sequential development. This is due to the influence of the harsher environment at the shelf edge.

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A single specimen of fossil worm from the Trenton Limestone, recovered from a quarry northeast of Quebec City, may represent a new form that cannot be directly compared with any other known fossil or recent worm. The specimen is incomplete and shows evidence of decay, but the remarkable preservation of the surviving portion shows well defined transverse lines, interpreted as segmentation, and traces of internal organs. There is, however, no evidence for either setae or parapodia. In certain respects the anatomy of this worm is reminiscent of the annelids, and in particular the oligochaetes, but any similarity may be superficial. Other fossil worms that have been interpreted as oligochaetes are reviewed briefly, and a redescription of the worm Protoscolex covingtonensis Ulrich from the Eden Group (Ordovician) of Kentucky is given.

E3B 543Middle Ordovician sediments of the St. Lawrence Lowland. eastern Canada, and its northeastward extension to S t -S i o n , arc subdivided into the numerous formations of the Chazy.