76 GEOLOGY OF THE FIRST DISTRICT. 
Tessella catena. (PI. 22, %. 39?) Carapace lamelliform, often broader than long, 4-24 longitudinal 
series of transverse strise, 10 striae in line. 
Fig. 39 is copied from a species, of which I found a few individuals adhering to a dried 
Alga from Stonington, Conn. It appears to belong to T. catena. 
Fragillaria. Free, carapace sim'ple, bivalve or multivalve {siliceous), prismatic; forming chains 
resembling fragile ribbons, resulting from the imperfect division of the carapace and body. 
Fragillaria pectinalis. (PL 22, fig. 40.) Striate corpuscles broad, 2 to 4 times longer than broad, 
swollen and lanceolate on the lateral side, ovary yellow, yiy to Jg 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 filaments 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 gene¬ 
rally much longer than wide. Very minute striae may often be distinctly seen on the edges 
of the frustules, as represented in our figure, but sometimes it requires a high magnifying 
power and skilful management of the light to render these apparent. 
The masses composed of these filaments dry to a glistening silvery mass, which is exceed¬ 
ingly fragile, and which is unchanged by fire or nitric acid. 
This species is not unfrequent in the fossil state, but the chains are then usually broken up. 
PI. 22, 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. 
hipunctata. It occurs abundantly at Detroit, Mackinaw, and West-Point. 
Fragillaria trionodis. Ehrenberg mentions this species as occurring 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. 
Meridion. Free, carapace simple, bivalve or multivalve {siliceous), prismatic, wedgeform ; forming 
fragile spiral chains which of ten appear like complete circles, and which result from imperfect 
spontaneous division. 
Meridion vernale. (PI. 22. fig. 42, a, b.) Corpuscles wedgeform, striate, anterior end truncate and 
dentate, polypidom spiral, often appearing perfectly circular, Jy to Jy line. M. circulare, Agardh. 
M. circulare, Kuetzing, (Linn. 1833, PI. 15, fig. 37.) 
This is one of the most beautiful of the fresh water infusoria, and excites great admiration 
in all who behold its elegant form and markings, under a good microscope. It occurs in 
immense quantities in the mountain brooks around West-Point, the bottoms of which are 
literally covered in the first warm days of spring, with a ferruginous colored mucous matter, 
about one quarter of an inch thick, which, on examination by the microscope, proves to be 
filled with millions and millions of these exquisitely beautiful siliceous bodies. Every sub¬ 
merged stone, twig, and spear of grass, is enveloped by them; and the waving plume-like 
appearance of a filamentous body covered in this way, is often very elegant. 
