INFUSORIA. 
241 
I once, on a cold day in October, noticed vast collections of this species which were enve¬ 
loped in a mucous covering, and which formed large cloud-like masses several inches in extent, 
investing aquatic plants, stones, etc. Each of these masses was crowded with millions of 
the siliceous shells of this species. 
Ehrenberg mentions C. asperum as a new species detected by him among the fossils from 
West-Point. I am ignorant of its distinguishing features. 
Achnanthes. Carapace simple, bivalve or multivalve, siliceous, prismatic, longer than broad; fixed 
by one end, pedicellate; pedicel oblique, ventral, always simple, opening in the middle of the 
body. Groups, resulting from increase by spontaneous longitudinal division, resembling chains, 
little banners, plates or ribbons. 
Achnanthes brevipes. (Pi. 42, fig. 12.) Corpuscles striate, curved in the middle, ends rounded on the 
dorsal and ventral sides; pedicel thick, shorter than the body. 
I first noticed this species on filaments of Conferva fracta from Providence cove (R. I.), 
and have since found it abundant on marine algae from Stonington (Conn.). Small specimens, 
differing I believe in no essential character, are also very abundant on aquatic plants in the 
Hudson riyer at West-Point. 
The Achnanthes longipes of authors may be a distinct species, but the distinction “ pedi¬ 
cel longer than the body,” appears to me to be founded on a character liable to much varia¬ 
tion. I saw specimens at Stonington having pedicels much longer than the body, yet they 
appeared to me to agree with E. brevipes in every other respect. 
Striatella. Carapace simple, siliceous; fixed by one end, longer than broad, or nearly square, 
obliquely pediculate in form of little flags ; corpuscles without openings in the middle, often 
forming zigzag chains by spontaneous divisions. {Stipitate Bacillarice.) 
Striatella arcuata. (PI. 42, fig. 13.) Carapace lamellar, nearly square, with three to seven longitu¬ 
dinal internal lines, transversely striate; polypidoms (flags) in form of ribbons, often curved; nine 
striee in line. Diatoma unipunctatum, Agardh, Greville, &c. 
This species occurs in vast quantities on filiform marine algae at Stonington (Conn.). It 
covers the plants in such profusion as to make them glitter in the sunbeams as if covered 
with crystals. The recent frustules are not flat, but slightly convex, and are usually marked 
with an internal nearly circular spot, which in my specimens was yellow, not rose-colored as 
usually described. Considerable variation in the width of the frustules occurs even in the 
same ribbon. Each plate is transversely striate, the alternate lines not quite reaching to the 
edge. I saw numerous specimens supported by long pedicels. 
Geol. 1st Dist. 31 
