VII] LITHOTHAMNION. 187 



specimens of Lithothaimiion 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, Rosanofif, 

 Kosenvinge, Kjellman and Solms-Laubach\ 



It is to be expected that such calcareous algae as Litho- 

 thamnion 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 repre- 

 sentatives. As agents of rock-building the coralline algae are 

 especially important. The late Prof linger- 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 Vieuna as a building stone^ 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 differ- 

 ences. 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 



1 Hauck (85). Kosanoff (66). Kosenvinge (93) p. 779. Kjelhnan (83) p. H8. 

 Solms-Laubach (81). '•' UnK'er (r,8). 



=* A microscopic section of the Vienna Leithakalk is figured in Nicholson and 

 Lydekker's Manual of PaUeontnlof/y (89) vol. ii. p. 1497. 



