PALEOBOTANY BELLY. 299 



resistant that they have left only illy defined impressions in the rocks, 

 or, if they had hard parts as in the so called calcareous algae, these, 

 while they are often abundant, as in the various limestones of 

 Paleozoic, Mesozoic, and Cenozoic age, add little to the knowledge 

 obtained from a study of existing forms. They are, however, of 

 great philosophic interest, since they point to the steps in the evolu- 

 tion of the first life forms and since also they are unquestionably 

 the ancestors of the land plants which appear in the Devonian records 

 and from which all of our existing terrestrial vegetation has been 

 evolved. A few of the more important fossil types will be briefly 

 enumerated. 



Proterozoic algae are represented by the large concentrically lami- 

 nated calcareous deposits known as Cryptozoon, as well as by other 

 forms referred by Walcott to the genera Weedia, Collenia, New- 

 landia, Kinneyia, Greysonia, etc. Thousands of obscure tracings in 

 the Paleozoic and later rocks have been described as fossil seaweeds, 

 but large numbers of these supposed fucoids have been shown to be 

 the casts of trails, burrows and similar tracks of worms, trilobites, 

 and other marine organisms, while others are rill or current markings 

 of nonorganic origin. Nevertheless, there is no reason for doubting 

 the presence of algae from the pre-Cambrian onward, particularly 

 when the supposed fossils show a carbonaceous residue as in the 

 genus Spirophyton of the middle Devonian. The genus Nemato- 

 phycus of the Silurian and Devonian includes gigantic forms with 

 their internal structure preserved, the silicified stems of N. logani 

 being sometimes several feet in diameter. Nematophycus has been 

 regarded as belonging to the Siphoneae or possibly the Phaeophyceae 

 (brown algae). 



Another undoubted member of the latter group is the upper De- 

 vonian genus Thamnocladus. The Diatomaceae (Bacillariaceae) , 

 whose siliceous frustules accumulate as oozes in the present marine 

 and fresh waters, occur as fossils from the lower Jurassic onward. 

 They are sometimes present in the Tertiary as relatively pure beds 

 many feet in thickness and made up of billions of tests of these tiny 

 forms, as in the Miocene Calvert formation of Maryland and Vir- 

 ginia, or in the Pliocene of California. The Chlorophyceae or green 

 algae include many doubtful fossil forms and numerous others that 

 are well authenticated, particularly in the somewhat unique group 

 of Siphoneae. Joints of the verticillate or tubular calcareous forms 

 occur in the older Paleozoic (Cambrian to Silurian), where they are 

 represented by the genera Ascoma. Primicorallina, Sycidium, Calli- 

 thamnopsis, etc. This type becomes exceedingly common at certain 

 Mesozoic and Cenozoic horizons, as in the Mediterranean Triassic 

 (Diploporella, Halorella, Triploporella) or the middle Eocene of the 

 Paris basin. 



