The probable mode of life of the graptolites (crustoids, dendroids and graptoloids) is reconstructed from a, logical interpretation of their rhabdosomal structures. The readaptation of sessile colonial organisms to meet the requirements of a freely-moving existence in the plankton is regarded as the controlling factor in the evolution of the graptoloids. It is suggested that the gradual reduction in the size of the colony in the dichograptids, biserial and uniserial graptoloids may have been a consequence of the undesirability of coloniality in the new environment, and that the graptoloids might

have attained individuality, disappearing into the plankton at the time of their supposed extinction.