A short review of the New Zealand Thalassinidea, and of known species in the family Laomediidae is given, with a generic key to adults of the laomediid genera Jaxea, Laomedia and Naushonia. A key is given for the only species of the genus Jaxea known in the adult form, viz. J. nocturna and a second species (J. novaezealandiae n.sp.) here described from New Zealand waters.

Adults of J. novaezealandiae were obtained from mud bottoms between 2 and 12 fathoms, from Hawke Bay on the east coast of the North Island to Port Underwood on the south side of Cook Strait. They are abundant in suitable areas of Wellington Harbour and taken there in association with Squilla armata and Echinocardium cordatum. In the life history of J. novaezealandiae six essential "trachelifer" larval stages and a non-obligatory seventh contrast with the usual three, but rarely four or five, stages in most Reptantia.

Keys are given to the larvae of five species of Jaxea (including three species known only from larvae), to all six essential larval stages of J. novaezealandiae, and to the first post-larval stages of J. novaezealandiae, J. nocturna, Naushonia crangonoides and N. portoricensis.