} 
Sketch of the Infusoria of the faniily Bacillaria. 103 
<a. Fracrenarn. 
_Free, carapace simple, bivalve or multivalve, (siliceous) pris- 
matic, forming chains resembling fragile ribbons, resulting Srom 
the imperfect division of the carapace and bod 
A. Fragillaria pectinalis. (Pl. 2, fig. 40.) Sitete, aresealin 
broad, 2 to 4 times longer than broad, Lidlioin and lanceolate on the 
lateral side, ovary yellow, ;4z to 3; lin 
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 
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 strie 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 tirtstoles, 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 F. pectinalis, to which species all the 
varieties occurring at West Point appear referable. 
Merrion. 
Free, ine be simple, bivalve or multivalve (siliceous) pris- 
matic, wedgeform, forming fragile spiral chains which often ap- 
pear like complete circles, and which result from tmperfect spon- 
taneous division. 
Meridion vernale. (Pl. 2, fig. 42, a,b.) Corpuscles wedgeform, 
striate, anterior and truncate and dentate, polypidom spiral, often ap- 
pearing perfectly circular, ,'; to zy line. M. circulare, age M. 
circulare, Kutzing, Linn. 1833, Pl. 15, fig. 37. ais Ba 
