184 Dr. G. C. Wallich on Desmidiacea from Lower Bengal. 



XXI. — Descriptions of Desmidincece from Lower Bengal. 

 By G. C. Wallich, M.D., F.L.S. 



[With two Plates.] 



The Desmidiacese which form the subject of the present and 

 following communications were obtained during the latter months 

 of 1855, in the neighbourhood of 'Raneegunge, at that period 

 occupied as a small military outpost, about 120 miles to the 

 north-westward of Calcutta. 



Nearly one hundred and forty distinct species (representing the 

 whole of the British genera, with one exception, namely Didy- 

 moprium), or rather more than two-thirds of the entire number 

 of British species, were collected by me, in this short space 

 of time, within an area not exceeding 100 square miles in 

 extent. This fact is worthy of record, inasmuch as it renders it 

 highly probable that a very important addition to our knowledge 

 of this class of organisms would result from a careful exploration 

 of the adjoining districts, and more especially of the vast tract 

 of alluvial territory which, commencing near this point, stretches 

 away towards the sea, and presents all the characters pre- 

 eminently adapting it for the habitation of the whole tribe of 

 freshwater Algse. 



As might be supposed from the cosmopolite nature of the 

 Desmidiacere generally, a large proportion of the species referred 

 to are identical with those already known to occur in Europe 

 and America. Many, however, are new; and these certainly 

 equal, if they do not actually surpass, any of the hitherto re- 

 corded forms in beauty and symmetry. Amongst the more 

 common s])ecies a remarkable amount of " divergence'^ from the 

 typical character is everywhere to be met with, — a circumstance 

 depending, no doubt, on those peculiarities of soil and climate 

 which, in Lower Bengal, are so favourable to the exuberant and 

 rapid development of the entire vegetable kingdom. Such pe- 

 culiarities operate, however, on the more minute tribes in a 

 special degree ; for whilst the higher orders of plants are sub- 

 ject only to the regularly recurring changes of a tropical re- 

 gion, and undergo no abrupt or violent transitions as regards 

 habitat, the humbler Alga? arc borne abroad, during their 

 sporangial state, to great distances and into positions differing 

 widely in nature from those wherein they were originally en- 

 gendered. The liability to variation arising from this cause 

 must necessarily be extreme; and, therefore, few situations could 

 bo found in which the limits attainable by speeitic divergence, 

 as occurring in these organisms, could more easily and satisfac- 

 torily be determined. 



It is almost impossible to arrive at a correct estimate of the 



