188 THALLOPHYTA. [CH. 



of a fossil conceptacle containing tetraspores is shown in fig. 

 37, C ; it is from a Tertiary species of Lithothamnion, described 

 by Friih from Montdvraz in Switzerland. 



1. Lithothamnion mamillosuvi Gumb. Fig. 32, A (i) and (ii). 

 (p. 155.) This species was first recorded by Giimbel^ from the 

 Upper Cretaceous (Danian) rocks of Petersbergs, near Maestricht, 

 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-, but probably in ignorance of the 

 existence of Glimbel's Cretaceous species. 



2. Lithothamnion suganum Roth. Fig. 37, B. The section 

 of this form given in fig. 37, B shows three oval conceptacles 

 filled with crystalline material. The two lower conceptacles 

 originally communicated with the surface of the thallus, but 

 as in recent species the deeper portions of the algal body 

 became covered over by additions to the surface, forming 

 merely dead foundations for new and overlying living tissues. 



The cells of the thallus have a breadth of 7 — 9yu, and a 

 length of 9—12/^. 



The specimen was obtained from a Lithothamnion bank, 

 probably of Upper Oligocene age, in Val Sugana^ in the 

 Austrian Tyrol. 



Numerous other species of Jurassic, Cretaceous and Tertiary 

 age might be quoted, but the above may suffice to illustrate the 

 general charactei^s and mode of occurrence of the genus. It is 

 important that the student should become familiar with the 

 Lithothamnion and Lithophyllum types of thallus, in view of their 

 frequent occurrence in crystalline limestone rocks and in such 

 comparatively recent deposits as those of upraised coral-reefs. 

 The coral-rock of Barbadoes and other West-Indian islands 



1 Gumbel (71) PI. ii. fig. 7, p. 41. ^ Hauck (85) p. 272. 



3 Eothpletz (91) PI. XVII. fig. 4. 



