AECHEK NEW SPECIES OP COSMAEIUM AND AETHE0DESMT73.* - 67 



vading rule, that in this Family the deeply constricted short forms pos- 

 sess spinous zygospores. But I shall take a future opportunity to draw 

 attention to a few cases of exceptional form as regards the presence or 

 absence of spines in the zygospores of the Desmidiacese. 



Genus — Aethbodesmtts (Ehr.). 

 Arthrodesmus tenuissimus (mihi, sp. nov.). 



Specific Characters : — Frond extremely minute ; segments sub-hex- 

 agonal, opposite lateral extremities acutely cuspidate, each upper angle 

 furnished on each front with a minute acute mucro, which four in the 

 fusiform end view stand out divergently. 



Locality : — Featherbed Bog, in Sphagnum pools, coating the moss. 



General Description : — Frond very minute, smooth, somewhat broader 

 than long ; constriction, a rounded or bluntly triangular sinus ; seg- 

 ments in front view about twice as broad as long, sub-hexagonal, the 

 sides somewhat concave, the opposite lateral extremities projecting 

 horizontally, acutely cuspidate, ends truncate or slightly concave, the 

 two upper angles at each front view furnished with a minute spine or 

 mucro, but which, in this view, being turned rather towards the observer 

 than divergent, each appears as a minute somewhat opaque thickening 

 at the angles ; frond in side view about twice as long as broad, oblong ; 

 ■constriction, a shallow emargination, ends broadly rounded, each fur- 

 nished at each opposite side with a single short acute divergent mucro ; 

 end view broadly fusiform, about twice as long as broad, the body bear- 

 ing on each side, near the acutely cuspidate extremities, two short acute 

 divergent mucrones. Zygospore unknown . 



Measurements : — Length of frond, 2^00 > Drea( ith, ^-^jj; at constric- 

 tion, 3^0 ; depth (thickness), ■$■£■$ <j of an inch. 



PL I., Fig. 50, front view; 51, side view; 52, end view of frond; 

 53, 54, dividing fronds ; 55, abnormal frond. 



Affinities and Differences : — As to the specific distinctness of this 

 minute form, there seems to me not the smallest doubt or difficulty. 

 There might, however, in the opinion of some be a question as to its 

 generic position; for it seems possible that the same conflicting views 

 which have been held by different observers as regards Arthrodesmus oc- 

 tocornis might also be held with respect to this new form. Arthrodes- 

 mus octocornis (Ehr., Hass., Breb., &c), was at one time placed in the 

 genus Xanthidium by Ehrenberg — Meneghini and Kiitzing considered it 

 a Micrasterias — Kail's looked upon it as doubtfully a Xanthidium — but 

 whatever be the proper position of that species, there can be little 

 doubt, I think, but that the present form belongs to the same genus, and 

 I should undoubtedly be disposed to consider that to be Arthrodesmus. 

 Indeed, the only question, as it appears to me, is whether it be Xanthi- 

 dium or Arthrodesmus. Taking Arthrodesmus incus or A. convergent, 

 as typical of the genus, there does, indeed, appear some dissimilarity 



