248 MR. A. W. WATERS ON FOSSIL LITHOTHAMNIA. 



distribution, principally in the North temperate and Arctic 

 zones. They grow at but moderate depths, and are appa- 

 rently fairly common on the west coast of the British Isles, 

 especially on the coast of Ireland. 



Lithothamnium belongs to the family Melobesiaceae of 

 the order Florideae or Rhodospermeae. A large number of 

 this order are lime-secreting; but these [Lithothamnium) 

 differ very materially from the Corallinae, since in the 

 former the carbonate of lime is deposited in and between 

 the cell- structure, while with the Corallinae the plant is 

 filiform, and the lime is deposited round this, entirely 

 incrusting it. I think the structural difference of the 

 Melobesiaceae from the Corallinae and other limestone- 

 secreting seaweeds is sufficient to separate them into two 

 entirely distinct classes, since the similarity of stony ap- 

 pearance is merely morphological, the deposition of lime 

 being so different. 



The reproductive organs are produced in conceptacula, 

 called by llosanoff cystocarpi. The spores, tetraspores, 

 and antheridia are known of many recent species. 



One of the microscopical sections, of which a drawing is 

 before you, shows some of the conceptacles of this fossil 

 cut through. There can, I think, be no doubt of their 

 true nature. There is also a body which I take to be a 

 tetraspore ; but I feel it to be far from certain : as the 

 tetraspores would contain but very little carbonate of lime, 

 they could be but imperfectly fossilized. The concep- 

 tacula are, in recent species, according to Rosanoff, free 

 from carbonate of lime. 



Giimbel, after saying how frequently only fragmentary 

 pieces are to be had, and not fine bunches, and how the 

 stems and branches of a species are subject to variation as 

 well as the external characteristics and the superficial 

 nature, although some are easily distinguishable by the 

 character of branching and superficial form, says, " but by 



