362 



GEOLOGY. 



one another in succession, from the plants up to the master forms of 

 the predaceous animals, there must doubtless have been many inter- 

 mediate species that are not now known. The defensive investitures of 

 the lower forms, not fully accounted for by the known Cambrian species, 

 are now much more nearly explained by the prevalence of the cephalo- 

 pods and the presence of fishes; and the armorings of these dominant 

 forms may have been defensive against their own kind. The fact that 

 nearly all vegetal and animal tissue perished, and is not even repre- 



Ftg 167. — Ordovician Graptolites. a, Dichograptus octobrachiatus (Hall); b, Diplo- 

 graptus pristis (Hall) (restored by Ruedemann); c, Tetragraptus bigsbyi (Hall); 

 \d) , Dichograptus logani (Hall) ; e, Climacograptus bicornis (Hall); /, Reteograptus 

 eucharis Hall; g, Didymograptus nitidus (Hall); h, Tetragraptus fruticosus (Hall); 

 i, Phyllograptus ilicifolius Hall; /, Phyllograptus typus Hall; k, Holograptus rich- 

 ardsoni (Hall). 



sented by organic deposits save in exceptional cases, probably signifies 

 that bacteria and allied forms that are concerned in the decomposition 

 of tissue were present in abundance. What are interpreted as fossil 

 bacteria are found a little later in some of the earliest vegetal tissues 

 that are well preserved, and what seem to be the characteristic effects 

 of bacteria on the tissues are also then recognized. This gives support 

 to the view that the very general destruction of the tissues of the higher 

 Ordovician animals and plants was the work of such destructive micro- 

 organisms. The paucity of the fossil record was therefore perhaps as 



