66 natural history society of dublin. 



Description op a new species op Cosmaeium: (Corda), and op 



Arthrodesuus (Ehe.). By "William Archer. 



(Plate I.) 



[Read June 5, 1862.] 



Family — Desmidiacejs. 



Genus — Cosmarium (Corda). 



Cosmarium pygmceum (mini, sp. nov.) 



Specific Characters. — Frond very minute, smooth, segments sub- 

 quadrilateral ; end view sub-elliptic, somewhat inflated at each side at 

 the centre. 



Locality. — Featherbed Bog and elsewhere in pools in Dublin moun- 

 tains ; not very rare ; on submerged plants, and in Sphagnum pools, 

 coating the moss. 



General Description. — Frond very minute, smooth, rather broader 

 than long ; constriction a minute linear acute notch ; segments in front 

 view rather more than twice as broad as long, subquadrilateral, outline 

 sometimes slightly irregular, ends straight or slightly curved; side 

 view scarcely twice as long as broad, constriction a triangular emargi- 

 nation on each side, segments orbicular ; end view sub-elliptic, with a 

 gentle gradual central protuberance at each side, extremities sub-conical, 

 rather abruptly rounded. Zygospore orbicular, smooth (without spines), 

 placed between the shortly deciduous empty parent fronds. 



Measurements : — Length of frond, ayy o- to T -^ T o ', breadth, j-jVo to 

 f oVtt ; depth (thickness), T - 7 J oo to ttV o °f an inch. 



PI. I., Figs. 45, 46, front view ; 47, side view ; 48, end view of 

 frond ; 49, zygospore. 



Affinities and Differences : — There is no danger of mistaking this for 

 any other species, except, perhaps, G tincium ; but from it, this form is 

 at once distinguished by its subquadrilateral, not broadly and regularly 

 elliptic, segments, and by its colourless, not faintly reddish, cell- wall. 

 I have met this form for two or three years, and I consider it is perfectly 

 distinct, nor is there any other species with which it need be contrasted. 

 There is a slight variation as to size within the limits above-mentioned. 

 I might remark that the general contour of the segments seems to me 

 to resemble in some degree those of Cosmarium biretum (Breb.), and 

 which to those acquainted with that species may serve to help out my 

 description ; but it would be simply absurd to institute any further com- 

 parison between them. 



The orbicular smooth zygospore without spines, seems further to in- 

 dicate an affinity with G. tinctum. This latter I have moreover found 

 to possess orbicular, not quadrate zygospores — thus, perhaps, pointing 

 to an affinity between these species more strongly. But the parent 

 forms are readily distinguished by the characteristics alluded to above ; 

 and I cannot fancy the possibility of their being confounded. Thus this 

 distinct little species forms an additional exception to the generally per- 



