The description and classification of new material from the Upper Cambrian of the Llano uplift in Texas has provided the basis for a review of the whole problem of the oldest cephalopods. In the light of these discoveries it is possible to evaluate more critically the Cambrian fossils that have formerly been assigned to the cephalopods. These fossils may be grouped as follows:The new genus and species Palaeoceras mutabile exhibits siphuncular bulbs, previously reported only in material from eastern Asia. From this new evidence, the bulbs are accepted as a feature of the family Plectronoceratidae, which is redefined accordingly. The other genera, Plectronoceras of the Cambrian and Multicameroceras and Sinoeremoceras of the early Ordovician, are briefly reviewed.Data concerning three Cambrian genera which are certainly cephalopods, but whose extant descriptions and illustrations are insufficient to determine their precise morphology and taxonomic position, are analyzed in detail. Shantungendoceras, which has been reviewed elsewhere, is probably Ordovician and not Cambrian in age. Angaroceras is regarded as a synonym of Ruthenoceras. What can be learned concerning the genera in this group is sufficient to determine that they may belong either to the Ellesmeroceratidae or Plectronoceratidae, but could not be members of any more advanced families.New material of Shelbyoceras shows clear septa, but no siphuncle, and an aperture with a sinus on the convex side. If a siphuncle is present, it is tiny and close to the same side. Either Shelbyoceras is an exogastric cephalopod without close relatives, or it is a septate shell without a siphuncle, and therefore not a cephalopod. The second interpretation is regarded as probably the correct one.Some Cambrian shells formerly assigned to the cephalopods have proved to be aseptate. One such form is described as Kygmaeoceras perplexum. Such fossils, which include the Hyolithidae and Tentaculitidae, are of uncertain position, because the shell offers no good evidence of the nature of the animal of which it was once a part.A stratigraphic survey shows that as cephalopods are traced from the Chazyan to older strata, the uniformity of form and structure increases. It is evident that a general structural pattern, based upon the combined features of the oldest families, the Ellesmeroceratidae, and Plectronoceratidae, can be accepted as archaic. Within this general pattern it is possible to select more specific features as reasonable primitive ones. The siphuncular bulbs of the Plectronoceratidae are regarded as truly archaic cephalopod features.In the light of this summary, it is evident that the cephalopods converge in the lower Canadian and Upper Cambrian to a reasonable uniform structural pattern. This pattern is so remote from that of the Lower Cambrian genera Volborthella and Salterella, that it is quite evident that these tiny shells have no close relationship with the oldest fossils which can be called cephalopods with certainty. They well may be included with the invertebrates of uncertain position.