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



The characters of the two genera, as given by Ralfs, are as 

 follows : — 



" Desmidium : filament fragile, elongated, triangular or qua- 

 drangular, regularly twisted ; joints bidentate at the angles. 



" Aptogonum : filament elongated, triangular or plane ; 

 joints bicrenate at the margins ; an oval foramen between the 

 joints." 



And again : " Certainly the state with triangular filaments 

 does at first sight appear closely allied to Desmidium Swartzii ; 

 but the large oval foramen between the joints is so remarkable 

 a cbaracter, that 1 must concur with Ehrenberg in placing it in 

 a separate genus*." 



Now in the Bengal forms the " foramina" are to be met with 

 indiscriminately in all the species and varieties ; that is, in the 

 genus described by Ralfs as Desmidium, and also in that which 

 must be referred to Aptogonum. The generic character founded 

 on this structure must therefore fall to the ground. 



In Aptogonum Desmidium, as figured {loc. cit.), as well as 

 in all the British specimens that have come under my notice, 

 the whole physiognomy of the plant tallies so exactly with 

 the characters of Desmidium, and the presence of a constriction 

 is so distinct as, in my liumble judgment, to leave no room for 

 doubt. On the other hand, no two forms can appear more dis- 

 similar in general outline than the species just referred to and 

 that recorded as Aptogonum Baileyi. For not only is all sem- 

 blance of constriction wanting, but the margins are so direct 

 and so nearly plane as to strike the eye at the first glance. The 

 figure of the American species, as given by Mr. Ralfs from a 

 drawing by Professor Bailey, presents no trace of marginal 

 crenature, and in this feature fails to conform to the generic 

 character assigned to it. But, from its unmistakeable identity 

 with the triangular form from Bengal, and likewise from what 

 is to be observed in the quadrangular variety from the same 

 locality, I cannot help thinking that the faint marginal irregu- 

 larity does in reality exist. In the Bengal forms it depends on 

 a large oblong portion of each free face of the joints being very 

 slightly raised above the surrounding surface. As seen, there- 

 fore, in difierent positions of the joint, these flattened elevations 

 appear broader or narrower according as the ends or broader 

 portion of the oblong strike upon the outline. 



On careful examination, it will be found that the spaces alluded 

 to under the name of " foramina " are to be met with in all the 

 filamentous forms the joints of which are united together by 

 intervening projecting cushions or processes placed at their 



* 'The British Desmidiea?,' Ralfs, pp. 60, 63 & 208, tab. 4, 5, & 32. 



