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.
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.
Species of Lithothamnion occur in the Mediterranean Sea, and are abundant in the arctic regions[353], while on the British coasts the genus is represented by four species[354]. Some large specimens of Lithothamnion and Lithophyllum are exhibited in one of the show-cases in the botanical department of the British Museum. For the best figures and descriptions of recent species reference should be made to the works of Hauck, Rosanoff, Rosenvinge, Kjellman and Solms-Laubach[355].
It is to be expected that such calcareous algae as Lithothamnion should be widely represented by fossil forms. In addition to the botanical importance of the data furnished by the fossil species as to the past history of the Corallinaceae, there is much of geological interest to be learnt from a study of the manner of occurrence of both the fossil and recent representatives. As agents of rock-building the coralline algae are especially important. The late Prof. Unger[356] in 1858 gave an account of the so-called Leithakalk of the Tertiary Vienna basin, and recognised the importance of fossil algae as rock-forming organisms. The Miocene Leithakalk, which is widely used in Vienna as a building stone[357], consists in part of limestone rocks consisting to a large extent of Lithothamnion.
Since the publication of Unger’s work several writers have described numerous fossil species of Lithothamnion from various geological horizons. A few examples will suffice to illustrate the range and structure of this and other genera of the Corallinaceae. In dealing with the fossil species it is often impossible to make use of those characters which are of primary importance in the recognition of recent species. The fossil thallus is usually too intimately associated with the surrounding rock to admit of any use being made of external form as a diagnostic feature. The size and form of the cells must be taken as the chief basis on which to determine specific differences. In the absence of conceptacles or reproductive organs it is not always easy to distinguish calcareous algae from fossil Hydrozoa or Bryozoa. In many instances, however, apart from the nature and size of the elements composing the thallus, the conceptacles afford a valuable aid to identification. An example of a fossil conceptacle containing tetraspores is shown in fig. 37, C; it is from a Tertiary species of Lithothamnion, described by Früh from Montévraz in Switzerland.
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1. Lithothamnion mamillosum Gümb. Fig. 32, A (i) and (ii). (p. 155.) This species was first recorded by Gümbel[358] from the Upper Cretaceous (Danian) rocks of Petersbergs, near Maëstricht, on the Belgian frontier. It was originally described as a Bryozoan. The thallus has the form of an encrusting calcareous structure bearing on its upper surface thick nodular branches, as shown in fig. 32, A (ii); in section, A (i), the thallus consists of a regular series of rectangular cells.
The specific name mamillosum has also been given to a recent species by Hauck[359], but probably in ignorance of the existence of Gümbel’s Cretaceous species.