GENERAL RELATIONS OF PLANTS TO TIME. 431 



envelopes of Diatoms (fig. 692). The most celebrated of 

 these Diatomaceous deposits is the so-called " Eichmond 

 Earth" of Virginia, which attains a thickness of thirty feet, 

 and is of Eocene or Miocene age. Another similar deposit 

 is the " tripoli " or " polir-schiefer " of Bohemia. The Des- 

 mids, unlike the Diatoms, have no hard covering, and their 

 absence in a fossil state is therefore not surprising. The 

 singular microscopic bodies which are known as " Xanthidia," 

 have, however, been regarded as referable to the Desmidice. 

 They have the form of minute spheres provided with radiat- 

 ing spines, and they have been detected in the flints of the 

 Chalk, and in the chert of the Devonian formation. 



Professor Martin Duncan has recently drawn attention to 

 the existence of minute tubular borings in shells and corals 

 belonging to the Silurian and Devonian periods, which he 

 regards as the work of unicellular filiform parasitic Algce, 

 and which he names Pateac/i/?/^, on account of their apparent 

 relationship to the recent Aclilya. 



A much more important group of Algce is that represented 

 at the present day by the so-called " Corallines " and " Nulli- 

 pores." These have the power of secreting calcareous matter 

 within their tissues, and they often attain a considerable size, 

 so that they are well adapted for preservation in the fossil 

 state. Nevertheless, no traces of NuUipores have as yet 

 been determined, with certainty, from the older rocks of the 

 earth's crust. On the other hand, great accumulations of 

 the remains of these stony Algce are found in the Tertiary 

 series of Europe. Thus, the so-called " Leitha-Kalk " of 

 Austria is largely made up of calcareous concretion - like 

 masses, which are undoubtedly referable to the NuUipores 

 (fig. 693). Moreover, if Mr Carter be correct in the view 

 that the singular microscopic bodies known as " coccoliths " 

 are really referable to the Nullipores {Mclobesict), then we 

 must admit for these singular Algse a high antiquity ; since 

 these structures occur abundantly in chalk, and are stated by 

 Glimbel to be present in almost all limestones, including 

 even those of the Lower Palaeozoic formations. 



Lastly, the ordinary marine Algce appear to be represented 

 in all the stratified formations, from at least the Lower 



