Major divisions of the Cephalopoda

Division of the Cephalopoda into Tetrabranchiata and Dibranchiata involves implications as to the nature of unknown soft parts of fossil forms. Factual and theoretical evidence combine to suggest that many of the features of the Dibranchiata may have developed in their ancestors included in the Nautiloidea, and that many of the features of Nautilus are derived, or possibly connected with, crucial changes in mode of life now known to have occurred in its ancestry. Use of similar divisions to separate forms with external versus internal shells also involves inferences; recent work has shown this boundary to be one difficult to draw in relation to primitive belemnites and some supposed bactritids. It is urged that the tripartite divisions of the Cephalopoda into Nautiloidea, Coleoidea, and Ammonoidea is in closer accord with ascertainable facts and avoids attributing some 3,000 extinct genera to a group characterized by the 4 gills observable only in 1 genus, the living Nautilus.