The collection obtained during the brief cruise of H.M.C.S. "Thetis" proved to be exceptionally rich in sponges; over one hundred and fifty species were procured, and approximately fully one half are either rare or new species.
The present paper deals with a little less than half of the Monaxonida, and includes nineteen new species, a number of others of great interest—hitherto only obtained by H.M.S. "Challenger"—and many other forms originally described by Dr. R. von Lendenfeld. The latter have been treated at some length, and have formed the basis of comparison between the types in the Australian Museum collection and the fragments received from Prof. A. Dendy, which were selected from the Lendenfeldian collection now in the British Museum. The material obtained enabled the writer to amend some of the descriptions and also the nomenclature of many of the exhibited specimens, as well as of those published in the "Catalogue of Sponges in the Australian Museum."
The new species herein described are as follows: Gellius reptans; Rhaphisia ramosa; Chondropsis syringianus; Esperella ancorina; Esperella cylindrica; Esperella textilis; Esperiopsis canaliculata; Esperiopsis ferruginea; Cladorhiza waitei; Phelloderma polypoides; Desmacidon porifera; Desmacidon hispidosa; Desmacidon stelligera; Desmacidon conulissima; Desmacidon ?arenosa; Dendoryx pumicea; Dendoryx fusca; Iotrochota arbuscula; Yvesia commensalis.