BY G. I. PLAYFAIR. 843 



identify Kiitzing's plant with P. tetraedricum Nag., but their 

 figures do not agree with one another nor even exactly with 

 Nageli's figures. Also Nageli gives the same two types to P. 

 trigomim and P. tetraedricum, relying for a specific characteristic 

 on the plane or tetraedral form of the cell. There is no biological 

 difterence, however, between plane and tetraedral cells, the latter 

 being developed often from the foxnner; and as the classification 

 is on the basis of outward configuration, it is extremely incon- 

 venient to have types of a mixed character. The occurrence of 

 biradiate and tinradiate fox^ms in the same type is well known in 

 the Desmid genus Staurastrum, and these are strictly analagous 

 to the plane and tetraedral forms of Tetraedron. I consider, 

 therefore, that NageH's T. iv.B, fig. la, and T. iv.B, fig.3a,b, should 

 be arranged under Tetr. trigonum, while T. iv.B, fig.lc, d, and T. 

 iv.B, fig. 3c, d, should be classed as 2'etr. regtilare (tetraedricum). 



Var. INERME, n.var. (PI. lix., f. 17). 



Cellula? plana?; lateribus leviter concavis; angulis levissime 

 inflatis; spinis nullis. Cell. diam. 14, crass. 6/i. 



Collector. 



The cells are plane, and have the concave sides and slightly 

 inflated angles of the type, but are without spines. 



Var. PAPILLIFERUM (Schrod.) mihi, forma. (PI. Iviii., f,20). 



Forma maxima tetraedrica; cellulse obesje; lateribus paullo 

 concavis; angulis latis, inflatis, papillis singulis prseditis. 



Cell. diam. 64/a. Collector, Rose's Lagoon. 



Cf. Polyedrium trigonum var. papilliferum Schroder, Ploner 

 Berichte, v., 1897, T. ii., f.6. In optical section, our form is the 

 exact same shape as Schroder's, from which it difi'ers in its tetra- 

 edral form, and very much larger size (diam. 12-15/i, — Schroder). 

 Both forms, with their hollow sides and inflated angles, naturally 

 fall under Tetr. regulars, though somewhat more plump than 

 figured by Nageli (P. tetraedricum). Cf. G. S. West's form. I.e., 

 sup7-a. The occurrence of a plane and tetraedral variety, in such 

 a distinct form as this, goes far to bear out my contention, that 

 there is no biological difference between them, but that both 

 may be found in the same species, 



