DEVONIAN AND OLD RED PERIOD. 



141 



microscopic plants (Diatoms and Desmids}. We find also in 

 the same siliceous material the singular spherical bodies, with 

 radiating spines, which occur so abundantly in the chalk flints, 

 and which are termed Xanthidia. These may be regarded 

 as probably the spore-cases of the minute plants known as 

 Desmidicc. 



Fig. 78. A, Trunk of Prototaxites Logani, eighteen inches in diameter, as seen in 

 the cliff near L'Anse Brehaut, Gasp6 ; B, Two wood-cells showing spiral fibres and 

 obscure pores, highly magnified. Lower Devonian, Canada. (After Dawson. ) 



The ' Devonian Protozoans have still to be fully investi- 

 gated. True Sponges (such as Astraospongia, Spharospongia, 

 &c.) are not unknown; but by far the commonest repre- 

 sentatives of this sub-kingdom in the Devonian strata are 

 Stromatopora and its allies. These singular organisms (fig. 

 79) are not only very abundant in some of the Devonian lime- 

 stones both in the Old World and the New but they often 

 attain very large dimensions. However much they may differ 

 in minor details, the general structure of these bodies is that 

 of numerous, concentrically-arranged, thin, calcareous laminae, 

 separated by narrow interspaces, which in turn are crossed by 

 numerous delicate vertical pillars, giving the whole mass a 

 cellular structure, and dividing it into innumerable minute 

 quadrangular compartments. Many of the Devonian Stromato- 



