342 W. H. Twenhofel — P re-Cambrian and 



structure which resembles the structure of the plaut. It 

 is possible that the filaments and thalli of the algae may 

 leave molds within the precipitated material. 



Terminology and Principle of Classification. 



The terms employed for the lime carbonate structures 

 which are formed in the manner outlined above are not 

 uniform. Garwood, Howe and others use the term thal- 

 lus, but that term had better be limited to its original 

 significance. It is true that in some cases the precipi- 

 tated lime carbonate may have the same general shape as 

 the thalli, but even in those cases there is considerable 

 doubt as to the propriety of calling the precipitated mate- 

 rial by the same name as the leaf -like structures of the 

 plant. Some writers have called these structures algae, 

 but to so term them has about the same application as 

 calling a worm boring a worm. Howe and others use the 

 term thallium with a suitable prefix for certain parts of 

 these structures, as hypothallium, perithallium, etc. 

 While appropriate, that term, however, has already been 

 used as the name of an element. As a suitable name for 

 these algal structures appears to be lacking and as one 

 is certainly needed, the writer proposes the word, coenop- 

 lase, from koinos, common, and plasis, formation. This 

 term is used in the present article. 



A ccenoplase from the very nature of its origin may, 

 or may not, show anything of the structure and charac- 

 teristics of the plants which are responsible for its devel- 

 opment and algae giving rise to coenoplases quite similar 

 in shape may be widely different in appearance and 

 general make-up. On the other hand it appears to be 

 reasonable to believe that algae belonging to the same 

 genus would develop coenoplases of so similar a form and 

 structure that little distinction could be made between the 

 deposits of different species and deposits of closely 

 related genera might also be apt' to be quite similar in 

 general appearance. If these assumptions be correct, it 

 follows that small constant differences in the shape, size 

 and structure of coenoplases are to be given a far greater 

 importance than would be the case were one dealing with 

 essential structures of animals and plants. It is on this 

 basis that the Carboniferous algae described in this paper 

 are referred to different genera. 



