186 



THALLOPHYTA. 



[CH. 



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, 



Fig. 37, A. Section of a recent Lithothamnion (after Eosanoff', x200). 

 B. Section of Lithothamnion suganum. Roth (after Eothpletz^, x 100). 

 G. A couceptacle with tetraspores from a Tertiary Lithothamnion (after 

 Fruh^, X 300). D. Sphaerocodium Bornemanni Roth, (after Rothpletz, 

 X 150). 



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. Con- 

 ceptacles 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. 



In the closely allied Liihophyllum 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'*, while on the British 

 coasts the genus is represented by four species'* Some large 



1 Rosanoff (66) PI. vi. fig. 10. 



3 Fruh (90) fig. 12. 



5 Holmes and Batters (90) p. 102. 



■' Rothpletz (91) PI. XVII. fig. 4. 

 * Kjellman (83). 



