In the Mediterranean there are true Nullipore reefs, which are interesting geologically as well as botanically. Walther[347] has described one of these limestone-banks in the Gulf of Naples which occurs about 1 kilometre from the coast and 30 metres below the surface of the water. Every dredging, he says, brings up numberless masses of _Lithothamnion fasciculatum_ (Lamarck), and _L. crassum_ (Phil.). Between the branches of the algae, gasteropods and other animals become completely enclosed by the growing plants, while diatoms, foraminifera, and other forms of life are abundant. Water percolating through the mass gradually destroys the structure of the algal thalli, and in places reduces the whole bank to a compact structureless limestone. The same author[348] has also called attention to the importance of _Lithophyllum_ as a constructive element in the coral-reefs off the Sinai peninsula. _Lithothamnion_ a typical genus of the Corallinaceae may be briefly described. _Lithothamnion._ Fig. 37. Philippi[349] was the first writer to describe this and other genera as plants. He gave the following definition of _Lithothamnion_: “Stirps calcarea rigida, e ramis cylindricis vel compressiusculis dichotoma ramosis constans.” The thallus of _Lithothamnion_ grows attached to the face of a rock or other foundation, and forms a hard, stony mass, assuming various coralline shapes. The exposed face may have the form of numerous short branches or of an irregular warty surface. In section (fig. 37, A.) the lower part of the thallus is seen to be made up of rows of cells radiating out from a central point, and the upper portion consists of vertical and horizontal rows of cells. The whole body is divided up into a large number of small cells by anticlinal and periclinal walls, and possesses an evident cellular as distinct from a tubular structure. Conceptacles containing reproductive organs are either sunk in the thallus or project above the surface. The two types of structure in a single thallus are shown in fig. 37, A, also a conceptacle containing tetraspores. [Illustration: FIG. 37, _A._ Section of a recent _Lithothamnion_ (after Rosanoff[350], × 200). _B._ Section of _Lithothamnion suganum_, Roth (after Rothpletz[351], × 100). _C._ A conceptacle with tetraspores from a Tertiary _Lithothamnion_ (after Früh[352], × 300). _D._ _Sphaerocodium Bornemanni_ Roth, (after Rothpletz, × 150).] In the closely allied _Lithophyllum_ the thallus is encrusting, and in section it presents the same appearance as the lower part of a Lithothamnion thallus.