One of the main objects of the study of animal distribution is to determine the probable areas of origin and dispersal—Osborn's Centres of Radiation—of the different families and genera of the various classes of animals. This is not an easy matter, and, as reference to any standard work on molluscs would show, the questions of the origin, in time and space, of the various families of molluscs, as also their relationships, are often severely left alone. The classification is, as a result, often quite arbitrary and does not show the genetic relationships of the different members. In dealing with families with a world-wide and, more particularly, a discontinuous distribution, it is a matter of grave doubt whether several of the families, as defined, are homogeneous units which originated in any one area of the globe and spread to other regions.