DI'ATOMA'CEIE (Xeo-Lat. nom. pl. from Siaroa4, tom(• severance. cliofrio to in, to cut throu•h., from Sid, din. through 71/Avfip, tentio. in, to cot 1. At) immense group of unicellular algae, estimated at 10,000 species, The cells are solitary or united in various ways into colonies that are frequently elaborately branched. Each cell is provilled with a siliceous shell in two halves, which lit closely together. These shells, vaned valves, are exquisitely sculp tured and make favorite preparations for the amateur microscopist. 3lany diatoms have power of movement, although the yells are never pro vided with cilia. They slide rapidly backward and forward in a way that is not well under stood.
The strueture and distribution of the cell contents in the diatoms suggest tile strlIctilre of dvsiiiiik (q.v.), hot the color is generally brown Cendoehroute't. ()wing to certain peculiarities of celbdivision, it is neve-soly that diatoms at certain periods pass into It resting stage. ill nhich the protoplasm ifrepare: fur a new period of vegetative acti\ ity. 'Hui: resting stage is called an auxospore. In some eases the cells change dire•tly lulu auxospores, but in at large proportion of forms the result from the fusion of the cell contents of two diatoms. This union of protoplasm doe: not always pre sent the characters of a sexual act, but it is generally believed to be a degenerate form of sexuality.
The diatoms are common in both fresh and salt waters, 'and constitute one of the chief sources of food-supply for the lower marine and la•ustrine animals, and through these for the higher organisms.
Their siliceous skeletons, which fall to the bottom. favor their ready entombment and pres ervation as fossils, and their lowly organization suggests the hypothesis that their remains may bawd in rocks of the Cambrian and Ordovi cian series. which contain the oldest foraminifera and radiolarians. They are not, however, posi tivel• known to occur in rocks of older than Upper Cretaceous age, which contain species that differ little from those now living, and that belong almost entirely to living genera. lu Tertiary time they were exceedingly abundant, and in many parts of the world their remains, with little admixture of other materials, form deposits of considerable thickness and economic importance. These deposits constitute an impor tant source of abrasive materials, and the prod ucts are known under such names as tripoli, dia tomaceous earth (qq.v), Richmond earth, elec iro silicon, etc. These diatom de posits are most common in Miocene rocks of both marine and fresh-water origin. in some rocks particular species of diatoms form the bulk of definite layers; different localities have their peculiar generic assemblages. The majority of Tertiary diatoms arc of species still living. Dia toms have been reported as having been found is the ash of English coal burned in Rome, but investigators, after the most careful search, have failed to detect them at home. See ABRA:,IVES.