ALLUVIAL DIVISION. 
239 
which it is often difficult to see any portion of the supporting plant. When the algae on which 
it grows are dried, they often have a greenish grey hue, from the presence of this parasite. 
It presents considerable resemblance to S. gailloni, Ehr. (Diatoma crystallinum, Ag.) and 
has, like that, minute striae on the edges ; but I have seen no allusion made in the descrip¬ 
tions of that species, to the fleshy projection which is so conspicuous in our species. 
A person who sees how abundant this species is in the recent state, will no longer wonder 
that in the lapse of years masses of infusorial shells should accumulate so as to form extensive 
strata. 
Synedra -? (PI. 42, fig. 2.) Frustules linear, strait, striate, truncate on the lateral side; ventral 
sides with a neck-like contraction near each end; ends rounded. 
This species is very abundant in fresh water near West-Point, often covering aquatic plants 
with a glittering envelope of crystal-like frustules. The individuals are perfectly linear, with 
truncate ends. When seen laterally, they show near their extremities a slight contraction, 
which forms a neck supporting the round of knob-like terminations. Minute striae may be 
seen as represented in the figure. 
This ■ species presents many points of resemblance both to Synedra ulna and Fragillaria 
rhabdosoma of Ehrenberg, but I cannot identify it positively with either. 
PoDosPHENiA. Carapace simple, siliceous, cuneiform; fixed when young by one end, afterwards 
often free; longer than broad; pedicel small, hemispherical, or venting. 
In PI. 42, fig. 3, is represented a species which agrees with the above generic characters, 
and which I therefore place here, although it is a fluviatile production, while all of Ehren- 
berg’s species are marine. It invests stones, etc. in small streams near West-Point with a 
yellowish green covering, which appears like a mere stain ; but which, when scraped off with 
a knife, is seen to be composed of excessively minute frustules resembling those of Gom- 
phonema, but which have no perceptible pedicel. 
Gomphonema. Carapace simple, siliceous, cuneiform; fixed upon a distinct filiform branching 
pedicel; dichotomous by spontaneous division. 
Gomphonema minutissimum, (PI. 42, fig. 4.) Smooth? corpuscles cuneiform, curved, clavate, y^-yto 
tV 
A minute species agreeing closely with Kiitzing’s figure of G. minutissimum (see Linnea, 
1833, fig. 43), occurs abundantly on various aquatic plants in the Hudson river at West-Point. 
It varies much in size ; fig. 4, a, shows the largest individuals, and fig. 4, h, the smaller ones. 
Gomphonema -. (PI. 42, fig. 5, a, b.) This large and beautiful species appears to be related to 
G. dichotomum and to G. geminatum, but I am unable to satisfy myself of its identity with either. 
I found it in vast quantities at the Island of Mackinaw, Straits of Michillimacinac, on a large 
cedar tree which was sunk in water ten or fifteen feet deep, and which was literally covered with 
large waving bunches of a yellowish white color, composed of the dichotomous filaments of this 
species, glittering with crystal-like particles. 
The individuals are striate, with one side cuneate, and showing at the broad end two inte¬ 
rior arcuate folds; the other side is elongated, obovate or clavate, with a central circular spot 
and longitudinal smooth portion. The pedicel is repeatedly dichotomous. 
