The thallus is usually encrusted with carbonate of lime; it is of a branched cylindrical form in the well-known _Corallina officinalis_, Linn. of the British coasts, of an encrusting and foliaceous type, in the genus _Lithophyllum_, and of a more coral-like form in the genus _Lithothamnion_. The reproductive organs occur in conceptacles, having the form of small depressed cavities in the thallus, or projecting as warty swellings above the surface of the plant. Asexual reproduction is by means of tetraspores formed in conceptacles resembling those containing the sexual cells. The Corallinaceae may be subdivided into the two families Melobesieae and Corallineae[344]. =Melobesieae.= Thallus encrusting, leaf- or coral-like; unsegmented. (_Melobesia_, _Lithophyllum_, _Lithothamnion_.) =Corallineae.= Cylindrical filamentous and segmented thallus. (_Amphiroa_ and _Corallina_.) The genus _Corallina_ is the best known British representative of the Corallinaceae. With other members of the group it was long regarded as a coralline animal, and it is only comparatively recently that the plant-nature of these forms has been generally admitted. _Lithophyllum_, _Lithothamnion_, _Melobesia_, and other genera of the Corallinaceae and some of the Siphoneae play a very important part in the building and cementing of coral-reefs. The pink or rose-coloured calcareous thallus of some of these calcareous algae or Nullipores imparts to coral-reefs a characteristic appearance. In some cases, indeed, the coral-reefs are very largely composed of algae. Saville Kent[345] describes the Corallines or Nullipores of the Australian Barrier-reef as furnishing a considerable quota towards the composition of the coral rock. Mr Stanley Gardiner, who accompanied the coral-boring expedition to the island of Funafuti, has kindly allowed me to quote the following extract from his notes, which affords an interesting example of the importance of calcareous algae as reef-building organisms. “It is quite a misnomer to speak of the outer edge of a reef like this (Rotuma Island) as being formed of coral. It would be far better to call it a Nullipore reef, as it is completely encrusted by these algae, while outside in the perfectly clear water, 10 to 15 fathoms in depth, the bottom has a most brilliant appearance from masses of red, white and pink Nullipores, with only a stray coral here and there.” Agassiz[346] has given an account of the occurrence of immense masses of Nullipores (_Udotea_, _Halimeda_ etc.) in the Florida reefs; his description is illustrated by good figures of these algae.