412 MR. G. 8. WEST ON 
Turner states *, with regard to his Nanthidium cosmariforme, 
that it “might just as correctly be called Cosmarium wanthidi- 
forme.’ Such species as Xanthidium armatum, Rabenh., and 
X. bifurcatum, Borge t, bear such a striking resemblance to 
Micrasterias anomala, Turner, that no doubt can exist as to their 
close relationship. 
It is highly probable that the large genus Staurastrum is a 
diphyletie assemblage, being derived partly from Cosmarium and 
most probably in part from Xanthidium. The direct transition 
from Cosmarium to Staurastrum is exhibited by the triangular 
varieties of Cosmarium biretum, C. costatum, C. pseudoprotuberans, 
Ze, 1, and by Staurastrum cosmarioides S, S. orbiculare, S.muticum, 
S. acarides, &e., not to mention the close resemblance existing 
between Cosmarium cylindricum and a six-ended form of Staura- 
strum Meriani, or the puzzling characters of such species as 
Staurastrum areolatum or S. Laconiense. A transitional form 
between Xanthidium and Staurastrum is illustrated by the tri- 
angular variety of Xanthidium antilopeum ||, the earliest indica- 
tion of the production of triangular forms along this line of 
descent being seen in Micrasterias and even in Evastrum 4. 
From the genus Staurastrum there is a small offshoot to the 
genus Dichotomum, but a line of descent also passes to Arthro- 
desmus, a genus which unquestionably derives its species partly 
from Staurastrum (although mostly from Xanthidium). Several 
species of Arthrodesmus with smooth cells and lateral spines 
resemble very closely such species of Staurastrum as S. dejectum, 
* Kongl. Sv. Vet.-Akad. Handl. Bd. xxv. no. 5, 1893, p. 99. 
t O. Borge in Bih. till K. Sv. Vet.-Akad. Handl. Bd. xxii. Afd. iii. no. 9, 
p. 16, t. 2. f. 24. 
+ Cosmarium biretum var. triquetrum, Bréb. in Mém. Soc. Sei. nat. Cher- 
bourg, iv. 1856, p. 130, t. 1. f. 9. C. costatum var. triquetrum, Nordst. in 
Ofvers. af K. Sv. Vet.-Akad. Fórh. 1875, no. 6, p. 25 (— C. abnorme var. trique- 
trum, Nordst., 1872). C. pseudoprotuberans var. trigonum, Nordst. “ Desm. 
Grönl.,” Z. ce. 1885, no. 3, p. 7, t. 7. f. 2. 
§ Staurastrum cosmarioides, Nordst., “ Desm. Bras.,” Vidensk. Medd. f. d. nat. 
Foren. Kjóbenhavn, 1869, p. 223, t. 4. f. 43, very much resembles a triangular 
variety of Cosmarium pyramidatum, Bréb.; consult also the remarks made by 
Beergesen in Vidensk. Medd, Foren. Kjóbenh. 1890, pp. 49-50, c. ff. 1-6. 
| X. antilopwum var. triquetrum, Lund. in Acta Soc. Scient, Upsal. 1871, 
p- 76, t. 5. f. 6; Wolle, Deem U.S. 1884, t. 22. ff. 1-3. 
€ Cfr. M. pinnatifida var. trigona, West in Journ. Bot. xxvii. 1889, p. 206, 
t. 291, f. 15; and E humerosum forma triquetra, Schröder in Forschungsberichten 
der Plöner Biol, Stat. 1897, p. 98, t. 2. f. 3. 
