168 Chlorophycece 



Cosmarium. The genus Dysphinctium can never be recognized 

 in a sound system of classification, as opinion must always remain 

 divided even upon many of the commonest forms that different 

 authors have included in it. Its characters are too indefinite and 

 artificial to be of any real systematic value. Similarly, Pleuro- 

 tceniopsis can never be established as a genus, as it would be a 

 small, polyphyletic assemblage, including a few strangely incon- 

 gruous species which occupy places far apart in the evolutionary 

 scheme of the genus Cosmarium. It must be remembered that 

 the primitive chloroplast of the Desmids is an axile one and 

 that the parietal condition has been independently acquired in 

 Cosmarium by a few scattered members of the genus. I have 

 already stated 1 that if the large genus Cosmarium is ultimately 

 split up, the sections will not have to be based upon single 

 characters alone, but upon combinations of characters of which the 

 external form will be the most paramount. Until more is known 

 concerning the distribution of species of Cosmarium, the genus is 

 best left in its entirety. 



The zygospores may be globose, angular-globose, cubical, or 

 almost of any outward form ; they may be smooth, scrobiculate, 

 furnished with simple or furcate spines of variable length, or 

 adorned with conical papillae. In a few species, such as G. melano- 

 sporum Arch., the spore-wall becomes perfectly black. 



As there are 250 British species of Cosmarium it is not easy to point out 

 the chief peculiarities of the genus. The largest species which occurs in 

 Britain, and also one of the rarest, is C. ovale Ralfs (length 182 188ju; 

 breadth 100107 /*). The smallest is C. subretusdforme W. & G. S. West 

 (length 7 '8 8 p ; breadth 6'2 6'5 p.). The commonest species found in the 

 ponds and ditches of lowland districts are C. Botrytis (Bory) Menegh., C. prce- 

 morsum Breb. (fig. 62 H), C. xnbcostatum Nordst., C. humile Gay, C. granattim 

 Breb. var. subgranatum Nordst. (fig. 62 C and D), C. abbreviatum Racib. and 

 several forms of C. Meneghinii Breb. In the bogs of moorland districts 

 C. CtK'tu-bita Breb. is usually abundant, and in upland Sphagnum areas 

 C. Ralfsii Breb., C. pyramidatum Breb., C. subtumidum Nordst., and others, 

 are fairly general. Some species, such as C. Holmiense Lund., C. unceps Lund., 

 C. subspeciosum Nordst. and C. Pokornyanum (Grun.) W. & G. S. West, are 

 usually found on dripping rocks, and C. Dovrense Nordst., C. microsphinctum 

 Nordst., and several others, prefer wet calcareous rocks. C. pygnnt'um Arch, 

 sometimes occurs in myriads amongst the leaves of submerged Sphagnum, 



Genus Xanthidium Ehrenb., 1834. The cells of this genus 

 are somewhat flattened as in most species of Cosmarium, and the 



1 G. S. West, 'Alga-flora of Cambridgeshire,' Journ. Bot. 1899, p. 115. 



