DEVONIAN AND OLD RED PERIOD. 



139 



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

 Desmiditz. 



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

 cliff near L'Anse Brehaut, Gaspe ; 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 investigat- 

 ed. True Sponges (such as Astrceospongia, Sphcerospongia, 

 &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- 

 porce also exhibit on their surface the rounded openings of 

 canals, which can hardly have served any other purpose than 

 that of permitting the sea-water to gain ready access to every 

 part of the organism. 



No true Graptolites have ever been detected in strata of 



