Sketch of the Infusoria of the family Bacillaria. 103 



Fragtllaria. 



Free^ carapace simple, bivalve or multivalve, [siliceous) pris- 

 matic, formi?ig chains resembling fragile ribbons, resulting from 

 the i'mperfect division of the carapace and body. 



1. Fragillaria pectinaUs. (PI. 2, fig. 40.) Striate, corpuscles 

 broad, 2 to 4 times longer than broad, swollen and lanceolate on the 

 lateral side, ovary yellow, y^^ to -^-^ line. 



The flat ribbon-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 fila- 

 ments are of a yellowish green color, and resemble flat ribbons 

 crossed by transverse 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 skillful 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. 



PI. 2, 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. 



2. Fragillaria trionodis. Ehrenberg mentions this species as oc- 

 curring in a fossil state at West Point. I am ignorant of its characters, 

 and may have confounded it with jF. pectinalis, to which species all the 

 varieties occurring at West Point appear referable. 



Meridion. 



F?'ee, carapace simple, bivalve or multivalve (siliceous) pris- 

 matic, wedgeform, forming fragile spiral chains which often ap- 

 pear like complete circles, and ichich result from imperfect spon- 

 taneous division. 



Meridion vernaJe. (PI. 2, fig. 42, a, b.) Corpuscles wedgeform, 

 striate, anterior end truncate and dentate, polypidom spiral, often ap- 

 pearing perfectly circular, ^^ to ^\j line. M. circidare, Agardh. M. 

 circidare, Kxitzing, Linn. 1833, PI. 15, fig. 37. 



