185 



Fossil Lithothamnia. 



(Pl. 23). 



Several spedes of fossil Lithothamnia have been described, 

 but they appear most frequently to be neatiy impossible to identify 

 with certainty, as the recognizable characters are lost. Of the 

 more delicate or little anastomosing forms only fragmentary pieces 

 are to be had, and of the larger anastomosing forms only the 

 more solid central portions appear to be left, so that on the whole 

 only the latter apparently are subject to an approximate determi- 

 nation of species. Besides, often only the organs of propagation 

 afford a certain characteristic. Therefore, it is in most cases very 

 difficult to know whether a form belongs to a living or is an ex- 

 tinct species. 1 ) 



An interesting note on Lithothamnia from deep cuttings of the 

 bottom at the mouth of the River Liffey is given by Prof. 0'Reilly 

 in Proc. Irish Acad. p. 223. He found several shells coated with 

 Lithothamnia from a depth of about 22 feet 6 inches below the 

 low-water mark of the river, and gives a sketch of the different 

 strata above the bed of shells and Lithothamnia. A few years 

 ago he sent me some of these specimens for identification. I then 

 considered them to belong partly to L. fasciculatum (Harv.) partly 

 to L. polymorphum. Later he kindly sent me a photography of 

 the specimens, and I am still of opinion that those determined as 

 L. fasciculatum (Harv.) belong to the same plant to which the 



x ) Cp. Unger, Leithakalk; Gumbel, Die sogen. Nullip. ; Waters, Notes 

 foss. Lithoth.; and Zittel, Palæont. II, p. 38. 



