These are among the most abundant of Cambrian fossils; most of them belong to the lower order of the class (Inarticulata), in which the shells are mostly horny and the two valves are not articulated together by a hinge. The horny-shelled types, Linnarssonia (I, 11), Lingulepis (I, 10), and Lin-gulella are of great interest, as they differ but little from certain brachiopods which still exist. The second order of Brachiopods, the Articulata, which have calcareous shells connected by an elaborate hinge, were more common in the Upper Cambrian. In subsequent periods they became vastly more numerous than the Inarticulata, and throughout the post-Cambrian divisions of the Palaeozoic their shells are found in incalculable numbers.
The Mollusca are already represented by their principal divisions. The Pelecypoda, or Bivalves (I, 15), are of very small size and found very scantily; their variety and relative importance have gone on increasing ever since Cambrian times. Gastropoda (I, 16, 17) occur in small numbers, especially in the Upper Cambrian. Fossils formerly referred to the Pteropoda, but now, generally regarded as worm-tubes, are among the most frequent of shells found in these rocks, but display no great variety. The Cephalopoda, which are the highest group of molluscs, are perhaps represented in the Cambrian by shells which are rare and minute in size, and almost confined to the uppermost part of the system; that is, assuming that the tiny Volborthella found in Russia and New Brunswick is really a cephalopod, but this is not certain.
The Cambrian fauna displays steady progress, being distinctly more advanced in the upper than in the lower division.