alg^:. 



1497 



Fig. 1364.— Section of 



Leitha-Kalk " 

 (Tertiary) from the neighbourhood of 

 Vienna, showing fragments of Nullipores 

 {Lithothamnion), enlarged three times. 

 (Original.) 



habit. The thallus of the Nullipores is highly calcareous and of 

 stony hardness, and it may be en- 

 crusting, botryoidal, stalactitic, or 

 bushy in form (fig. 1363). As re- 

 gards the microscopic structure of 

 the calcified thallus, the outer layer 

 is formed of hexagonal or quad- 

 rate cells of very small size, while 

 the cells of the interior are oblong 

 in shape. The fructification con- 

 sists of " cystocarps," scattered 

 through the thallus, and either im- 

 mersed or projecting as tubercles. 



As regards the distribution of 

 the genus in time, the remains of 

 species of Lithothamnion are found 

 in vast numbers in various Tertiary 

 formations, often giving rise to 

 massive limestones, such as the 

 "Leitha-Kalk" of Austria (fig. 

 1364), the " Nullipore-limestone " 

 of Algeria, and others. Remains 



of Lithothamnion are also found in the Cretaceous and Jurassic 

 rocks, and they have been said to 

 occur in rocks as old as the Car- 

 boniferous Limestone. The fossil 

 forms of Lithothamnion do not 

 exhibit any marked structural dif- 

 ferences as compared with the ex- 

 isting types of the genus. 



Class VI. Charace^. — This 

 class comprises a number of fresh- 

 water or brackish-water Algae, of a 

 green colour, and remarkable for 

 having lateral appendages corres- 

 ponding with leaves. The thallus 

 consists of a central stem giving 

 off whorls of leaves at intervals, 

 which in turn give off secondary >^/*(£ece 

 leaflets. The male reproductive ?i?^ h en if ge i ^,. Cent , ral p ° rtion of a 



1 leaf with an anthendmm (a) and a carpo- 



Organ has the form Of a Spherical gonium (s) ; c, Crown of cells at the apex 



,r r \ j i i_ °* t " e car Pogonium ; /3, Sterile leaflets ; 



SaC (fig. I365, a), and has been p t Large lateral leaflet near the fruit; 



Sometimes Spoken Of as the " glo- P"> P"> Leaflets attached to the base of the 



1 i ?} -~n, r t j carpogonium. (After Sachs.) 



bule. The female reproductive 



organ or "carpogonium" — sometimes spoken of as the "nucule" 



Fig. 1365.— Part of the thallus of Chara 



