Palaeontologie. — Algae. 173 



plus rapprochees les unes des autres que ne le sont Celles de cette 

 derniere espece. 



Pour des raisons analogues, on ne saurait non plus le confondre 

 avec le Dict. Remauryi Zeiller, chez lequel, en outre, les pinnules 

 alternent avec des lobes arrondis equivalant ä autant de pinnules 

 tres reduites, ni avec le Dict. Sarrani Zeiller, dont les nervures 

 secondaires demeurent indivises jusqu'ä leurs sommets. 



Le nombre des especes de Dict. recueillies jusqu'ä present ä 

 Tonkin, se trouve porte ä six, d.oni(\wd.X.x^: D. Nathorsti.D. Fiichsi, 

 D. Remauryi et D. Sarrani ont ete decrites par Zeiller dans son 

 grand ouvrage sur la flore rhetienne du Tonkin. Jongmans. 



Smith, G. M., The Organization of the colony in certain 

 four-celled Algae, (Trans. Wisconsin Acad. Scie. Arts Letters, 

 XVII. 2. p. 1165-1220. Fl. 85-91. 7 Textfig. 1914.) 



The possible combinations of the four-celled coenobic Algae 

 can be arranged in the following table. 

 L Cells Isoaxial. 



1. Linear colonies. 



2. Plane colonies: Tetracocciis botryoides West. 



3. Pyramidal colonies: Coelastnim microporum Naeg. 

 II. Cells Heteraxial. 



1. Coplanar series: 



A. Concurrent forms. 



a. Coliniar colonies. 



b. Non parallel colonies: Pediastrum Boryanum (Ynrp.) 

 Meng., Crucigenia (Kirch.) W. et G. S. West, Tetra- 

 strum Staurogeniae/orme (Schröd.) Chodat. 



B. Nonconcurrent forms. 



a. Parallel colonies. 



b. Nonparallel colonies: Crucigenia rectangularis {^a.eg.) 

 Gay. 



2. Noncoplanar series. 



A. Concurrent forms. 



B. Nonconcurrent forms. 



a. Parallel colonies: Tetradesmus wisconsinensis Smith. 



b. Nonparallel colonies. 



In the present paper a form from each of the main classes 

 is taken up and discussed both with respect to the variations that 

 normally occur in the coenobe, and to the modiiications that can 

 be produced in the cellular arrangement by the altered environ- 

 mental conditions. 



The study of the variations occurring within the limits of any 

 particular species naturally leads to a discussion of the doctrine of 

 polymorphism. The historical side of the question has been tho- 

 roughly reviewed by both Grintzesco and Chodat and refe- 

 rence may be made to the work of these authors. The earlier view, 

 expressed by Kützing, that there could be a change in algae as 

 great as a passage from one genus to another, has been more and more 

 restricted untill the recent polymorphists believe that this Variation 

 is confined to a few species. The change in the concept of poly- 

 morphism has been due to more accurate methods of study, notable 

 the application of the pure culture methods first used by Beye- 

 rinck. It is noteworthy, that the most agressive of the modern 



