OF THE FAMILY BACILLARIA. 121 



ARTHRODESMUS. 



Free, carapace simple, vnivalve, compressed in the form of a 

 plate, or a compressed band, articulated hij spontaneous division. 



1. ArtJirodesmiis quadricandatus. (Fig. 17, PL I.) Corpuscles ob- 

 longs straight ; chain or polypidom of four to eight individuals formed 

 by imperfect spontaneous division, four horns, (the middle corpuscles 

 rounded at the ends, the others with a liorn at each end.) Size of 

 corpuscles yl^" to -^^ line ; chains -^^ line. Scenedesmus magnus, 

 Meyen. 



This species, as it occurs in this country, is composed of from 

 four to eight green elliptical corpuscles arranged in a thread or 

 chain, usually having horns only on the two extreme corpuscles, 

 but not unfrequently similar projections may be seen on the mid- 

 dle ones also. Meyen, who considers this as a plant, states that 

 in spring it contains starch globules.* It is extensively diffused, 

 both in Europe and in this country ; I have noticed it as far 

 west as Ouisconsin, and south to Virginia. It is abundant in 

 ponds near West Point, N. Y. 



2. Arthrodesmus cuius. (Fig. IS, a, b, PL I.) Corpuscles green, 

 oblong, alternating in a right line, by spontaneous division. 



The small green bodies represented in our fig. 18, occur in the 

 well at Fort Putnam, West Point. They appear to belong to 

 this species. 



ODONTELLA. 



Free, carapace simple, univalve, compressed, form of flattened 

 articidated ribands, often pierced, produced by imperfect sponta- 

 neous division, articulcdions united by small projections. 



I have not satisfactorily determined any American species of 

 this genus, although I have occasionally seen small filaments 

 which agreed tolerably well with the generic character. 



MICRASTERIAS. 



Free, carapace simple, univalve, compressed, groups of a num- 

 ber of individuals in form of a flattened star, produced by imper- 

 fect spontaneous division. 



* Meyen, Pflanzen Pliysiologie, Vol. III. p. 437. 



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