The development of the cephalothoracic grooves, orientation of the chelae in life (correlated with burrowing), and modifications of the sexual characters of the males are the main attributes utilized in deducing the probable phylogeny of the Parastacidae. Geocharax, in which the cephalothoracic grooves are separated and clearly defined, the chelae are orientated in oblique planes, and the male genitalia are simple, is considered to be the most plesiomorphic genus: the species, which are only moderate burrowers, occur in the Bass Strait Basin of south-eastern Australia. The Parastacidae are considered to have originated in this one area of Australia, and to have dispersed from there to the other present day extra Australian regions of the southern hemisphere, each of the four extra Australian genera being the apomorphic sister-group of a genus occurring in this Australian subregion. The new genus Gvamastacus, with two included new species, is characterized by the extraordinarily large genital papillae of the male, and the eniarged pleura of the second abdominal segment of the female. The cephalothoracic grooves and lines are clearly defined and almost as well developed as in Geocharax.