244 BESCEiPTiON OF \Tolygastrica. 



which give a serrated outline to the chain. Common in fresh water 

 in South Carolina, Georgia, and Florida. 



Genus Miceasxerias (Ag.) — Frond simple, (binate only when 

 dividing), deeply divided into two, lobed segments ; the lobes incise 

 dentate, (rarely only bidentate), and generally radiant. 



In two species sporangia have been detected ; they are large, glo- 

 bular, and furnished with short spines, which at first are simple, but 

 subsequently branched at the apex. In the perfect state they are 

 particularly interesting from their resemblance to the fossil " X.an- 

 thidia,^'' of Elirenberg and others. 



The orbicular, plane, and deeply incised fronds will distinguish 

 Micrasterias from all other genera in this family. In Euastrum, the 

 only one with which it can be compounded, the fr-onds are oblong, 

 and the lobes are not incised. 



This genus is not equivalent to the one so named by Ehi'enberg. 

 (See the genus Pediastrum.) 



* Frond circular ; segments five-lohed ; lobes a/pproximate, the end 

 lobe narrow. 



M.. denticulata.=Euastriirn Rota, (Ehr.) — Large, orbicular, smooth; 

 lobes cuneate, dichotomously divided, the ultimate sub-divisions 

 truncate-emarginate with rounded angles ; the end lobe, the narrowest, 

 is simply emarginate. (P. 2 f. 121, 122, 123, and Sporangium, 

 P. 13, f. 22.) 



The process of conjugation appears similar to what takes place in 

 Staurastrum dejectum ; the contents of both fr-onds unite, and form a 

 globular sporangium enclosed in a fine membrane, and its snrface 

 gradually acquires scattered, stout, elongated spines, at first simple, 

 with their apex obtuse, but afterwards forked or trifid ; and, 

 finally, further branched, and frequently more or less re-curved. The 

 sporangia are of considerable siije, — a necessary consequence of the 

 union of the contents of both fronds. (P. 13, f. 22.) 



The truncate ends of the sub-divisions distinguish this species 

 from M. rotata. Length 1-1 13th. 



M. rotata. — Orbicular, smooth ; lobes dichotomously incised, ulti- 

 mate sub-di\dsions bidentate. It differs fr'om M. denticidata, in 

 having the ultimate self-divisions dentate. Length 1-9 1th. Breadth 

 1- 104th. 



