OF THE FAMILY BACILLARIA. 151 



bij imperfect spontaneous divisio7i of the body, and perfect division 

 of the carapace. The chains have spontaneous motion. 



Tessella catena. . (PI. II, fig. 39?) Carapace lamelliform, often 

 broader than long, 4-24 longitudinal series of transverse strife, ten 

 strias in y^^y of a line. 



Fig. 39 is copied from a species, of which I found a few indi- 

 viduals adhering to a dried Alga from Stoninglon, Conn. It 

 appears to belong to T. catena. 



FRAGILLARIA. 



Free, carapace simple, bivalve or midtivcdve, [siliceous) pris- 

 matic, forming chains resembling' fragile ribands resulting from 

 the impe?fect division of the carapace and body. 



1. Fragillaria pectinalis. (PI. II, fig. 40.) Striate, corpuscles 

 broad, two to four times longer than broad, swollen and lanceolate 

 on the lateral side, ovary yellow, y^ to -^-^ of a line. 



The flat riband-like filaments of this species are very common 

 in ponds and slow running streams near West Point, and they 

 often form masses as much as a square foot in extent. The 

 filaments are of a yellowish gTcen color, and resemble flat ribands 

 crossed by ti'ansverse parallel lines. Great variety occurs in the 

 size and form of the frustules, but they are generally much longer 

 than wide. Very minute striae may often be distinctly seen on 

 the edges of the frustules, as represented in our figure, but some- 

 times it requires a high magnifying power and skilful manage- 

 ment of the light to render these apparent. 



The masses composed of these filaments dry to a glistening 

 silvery mass, which is exceedingly fragile, and which is un- 

 changed by fire or nitric acid. 



This species is not unfrequent in the fossil state, but the chains 

 are then usually broken up. 



PL II, fig. 41, represents a variety (?) of this species, with very 

 narrow frustules, each of which, when living, was marked with 

 two yellowish spots, (ovaries ?) Perhaps this is F. bipunctata. 

 It occurs abundantly at Detroit, Mackinaw and West Point. 



