VIl] ZONATRICHITES. 129 



trace of the organism has been left, but only the canals by 

 which it penetrated the calcareous or bony skeleton. In some 

 of the examples of Palaeachlya figured by Duncan there appear 

 to be numerous spores in some of the sections, but it is 

 generally a very difficult and often an impossible task to dis- 

 criminate between the borings of fungi and algae in fossil 

 material. 



Fig. 27 B, which is copied from one of Bornet and Flahault's 

 drawings, represents a piece of Solen shell riddled with small 

 canals made by the organism which has been named by the 

 French authors Ostracoblabe implexa, and regarded by them as 

 a fungus. Fig. 27 C represents a small piece of the vegetative 

 body of Ostracoblabe obtained from a decalcified shell. In 

 endeavouring to determine the organism which has produced 

 borings in fossil corals or shells, it must be borne in mind that 

 some forms of canals or passages may have been the work of 

 perforating sponges, but these are larger in diameter than those 

 made by algae or fungi. By some writers' the tubular cavities 

 in shells have been referred to true algae, but others consider 

 them to be of fungal origin. 



As an example of a fossil alga referred to the Cyanophyceae, 

 the genus Zonatrichites^ may be quoted. Bornemann, who first 

 described the specimens, points out the close resemblance in 

 habit to some members of the recent Rivulariaceae. 



Zonatrichites. 



The author of the genus defines it as follows : — 



"A calcareous alga, with radially arranged filaments, forming hemi- 

 spherical or kidney-shaped layers, growing on or enclosing other bodies. 

 Parallel or concentric zones are seen in cross-section, formed by the 

 periodic growth of the alga, the older and dead layers serving as a foun- 

 dation on which the young filaments grow in radially arranged groups." 



The nodules which are apparently formed by species of this 

 genus occur in various sizes and shapes ; Bornemann describes 

 one hemispherical mass 8 cm. broad and 4 cm. thick. In some 



1 E. G. Wedl (59). Good figures are given in this paper. 

 ' Bornemann (86), p. 126, Pis. v. and vi. 



