The type species of the genus Polymastia Bowerbank, 1864 is Spongia mamillaris Müller, 1806. For almost two centuries the type specimen was ignored, but it was recently rediscovered in the Zoological Museum in Copenhagen (Denmark). This type specimen is here redescribed. It corresponds to specimens from the Swedish west coast collected recently close to the type locality and it does not correspond to the common Polymastia species from the NE Atlantic coast (North Sea, Channel Sea, British coasts) erroneously called "mamillaris" since Johnston (1842) and Bowerbank (1864). This common Polymastia corresponds to Polymastia (Spongia) penicillus (Montagu, 1818), according to a reexamination of a specimen from Montagu (BMNH 18.104.22.168 deposited in BMNH in the Dr Grant Cabinet). The main differences between P. mamillaris and P. penicillus are the number of layers of the cortex, the type of free spicules present in the choanosome which are intermediary tylostyles in P. penicillus versus ectosomal tylostyles in P. mamillaris, the shape of the spicules cylindrical in P. penicillus versus fusiform in P. mamillaris and the thickness of the spicules which vary from 4 to 12 µm in P. penicillus versus 8 to 24 µm in P. mamillaris.
Porifera, Polymastia, Demosponges, Hadromerida, type specimen.