156 THALLOPHYTA. [CH. 



Bactryllium from the plant to the animal kingdom ; he points 

 out that the specimens are too large for diatoms, and moreover 

 that they are asymmetrical in form and possessed a calcareous 

 and not a siliceous shell. He would place the fossil among 

 the Pteropods, comparing it with such genera as Guvierina 

 and Hyalaea. In view of Stefani's opinion we cannot attach 

 any importance to this supposed diatom, especially as it has 

 generally been regarded as at best but an unsatisfactory genus. 



B. CHLOROPHYCEAE (Green Algae). 



Thallus unseptate, having the form of a vesicle or a variously 

 branched coenocyte, wKich may or may not be encrusted with 

 carbonate of lime, or of filaments composed of cells containing a 

 single nucleus, or of cells in which more than one nucleus 

 occurs; in other instances consisting of a plate of cells or 

 a cell-mass. Asexual reproduction by zoospores and other 

 reproductive cells; sexual reproduction by means of the con- 

 jugation of similar gametes or by the fertilisation of a typical 

 egg-cell by a motile spermatozoid. 



This family of algae is represented at the present day 

 by numerous and widely distributed marine and fresh-water 

 genera, as well as by genera growing in moist air or as 

 endophytes in the tissues of higher plants^ 



Seeing how very few fossil forms have been described which 

 have any claim to inclusion in this subdivision of the Algae, 

 it is unnecessary to enumerate or define the various families 

 of the Chlorophyceae. It is true that many species have been 

 figured as examples of different genera of green algae, but few 

 of these possess any scientific value. There is, however, one 

 division of the Chlorophyceae, the Siphoneae, which must be 

 treated at some length on account of its importance from a 

 palaeobotanical and geological point of view. 



1 The Chlorophyceae have recently been exhaustively dealt with by Wille 

 (97) in Engler and Prantl's Pfianzenfamilien. 



