DIATOMACE^E are clearly made distinct by the flinty fronds singular striation, and absence of green color ing matter. Agardh asserts that many of these organisms have as much affinity with the min eral kingdom as with the vegetable, being in fact vegetable crystals, bounded by right lines and collected into a crystalliform body, and having no other difference from minerals than that the individuals have the power of agair separating from each other. As in the case of the desmidieae, there are solitary species, anc others grouped so as to form lines and mem- branes. In some, the production of new plants from spores presents the same dissimilarity be- tween the young and the adult forms. There are also numerous genera which can be accu- rately distinguished not only by the difference of form or outline, but by their own peculiar striations, markings, and dots. In both the single and the associated species there is a dis- tinct pellucid peduncle or footstalk. This is sometimes considerably dilated above, or else forked, sometimes repeatedly. In this case each frustule remains attached, the base dila ting as may be required. This arrangement gives a fan-like appearance of great beauty. Echinella flabellata, a fan-like marine diatom. But in the thread-like species it is only the corners that remain attached; as no stem or footstalk is visible here, it has been conjectured that it exists only in those plants which have grown from spores or in the seedling forms. Certain channels or apertures are so arranged as to convey the water to the inner cellular membranes, and thus to afford nutriment. The same curious conjugation to be seen in other ajgao has been detected in the diatom acese by Thwaites, and has been confirmed by Berkeley and Broome. It is computed that vast areas of solid earthy matter are due to the growth presence, and decay of these minute organisms Many of the most beautiful are found in the guano of commerce, doubtless swallowed in s food of birds, and still remaining in perfect preservation. In the United States, masses of several inches in thickness are found on the ns of ponds, composed of myriads of these rgamsms, which on being exposed to desicca- tion become as white and friable as chalk. Even it bogs and meadows abound with them The polishing powders sold under the name of tripoli composed of these natural silicious frag- ments. The soundings on the shores of Victoria Barrier, Australia, in water whose average depth is 1,800 ft., were found by Dr. Hooker to be invariably charged with diatomaceous re- mains. These fossil species are often so identi- cal with recent ones, that it would be scarcely too extravagant to admit the assertion of Ehren- berg, that species are to be found in a living state in situations where they have been propa- gated from times far anterior to the existence of man. The United States are rich in the diatomaceffl, both fossil and living. In the tertiary infusorial stratum of Eichmond, Va., Ehrenberg detected 20 genera and 46 species, of which all were also European excepting two. This group of American forms is of peculiar interest, because the strata at Richmond are de- cidedly of marine origin, and consequently give at once a general view of these marine micro- scopic forms along the North American coast. Of the perfectly free diatoms we have many species of namculacece remarkable for beauty, symmetry, or delicacy, or else for their stria- tions. The largest, most common, and most easi- ly distinguished is namcula viridis, of an oblong outline, found in every ditch and pond. It can be detected in great abundance in the ashes of peat, and in the deposits of infusorial earths. Its length is about -fa of a line. Several of a sigmoid outline are very remarkable for the delicacy of their stria3, of which may be men- tioned pleurosigma Baltica, P. hippocampus, but more particularly P. angulata. The lines of striation upon NitzscJiia sigmoidea are about TooVinr of an inch apart. In fragilaria we have long threads of frustules adhering with considerable firmness at their commissures; but in diatoma they adhere only at a single point, so as to form curious chains of divided or separated joints. Prof. Bailey describes 'bacillaria paradoxa as a very interesting spe- cies, presenting by its curious motions and its paradoxical appearance an object well calcu- lated to astonish all who behold it. At one moment the needle-shaped frustules lie side by side, forming a rectangular plate ; sudden- ly one of the frustules slides forward a little way, the next slides a little also, and so on hrough the whole number, each, however, retaining a contact through part of its length with the adjoining ones. By this united mo- ':ion the parallelogram is changed into a long ine ; then some of the frustules slide togeth- er again, so that the form is then much like a banner. Similar motions are constantly go- ng on, and with such rapidity that the eye can scarcely follow them. The cause of this motion s wholly unknown, but it is probably mechani-
- al and not vital. Mr. Smith, in his work on
he diatomacea), estimates this motion as be- n S ^iff m ch per second. In meridion vernaU we have one of the most beautiful of the fresh- water diatoms. It consists of spiral or helicoi- lal chains, to perceive which the specimens must be tilted on edge. It occurs in immense quan- ities in mountain brooks, covering every sub-
