DIC^LUS. 



DICTYOCHA. 



Sab-Tribe 4. Connecting membrane lubpersistent ; frustules after 

 aelf-divuion united into a zigzag chain. 



6 Genera 



JMoloma (jig. 14) . . 4 specie*. 



Onmmatopkora . . 2 species. 



TuMlaria . . . 2 specie*. 



AmpMitrtrai ... 1 species. 



Mulp\ia (Jig. 15) . . 4 specie*. 



Jilkmia .... 2 species. 



Sub-Tribe 5. Connecting membrane subpersiatent as a siliceous 



annulus; frustules after self-division united into a cylindrical 

 filament. 



8 Genera 



Podotira . . . . . 2 species. 

 Melotira (Jig. 1) . . . species. 

 Orthotira 



Tribe II. Frustulee invested with a gelatinous or membranaceous 



envelope. 

 " Sub-Tribe 6. Frond indefinite ; mammillate ; frustules scattered. 



1 Genera r 



MattogMa .... 3 species. 



Sub-Tribe 7. Frond definite; compressed or globular; frugtulea 

 scattered. 



* 



2 Genera 



JKdeieia (fig. 17) . . . 8 specie*. 

 SerteUyia .... 1 species. 



Sub-Tribe 8. Frond definite ; filamentous ; frustules in rows. 



3 Genera 



fncyonema .... 2 species. 



CoiMonmo .... 4 specie*. 



Sckizonema (fig. 18) . . . 16 species. 



Sub-Tribe 9. Frond definite, filamentous ; frustules fasciculated. 

 1 Qenns 



Homtfdadia.* . . . . 3 species. 



(Smith, Synopii* af Britith Diatomacea, vols. i. & ii. ; Meneghini, 

 On tke Animal Mature of Diatomea, translated by Ray Society; 

 Kutiiug, Specie* Algarum ; Siebold, On Unicellular Planit and 

 AnimaU, translated in Quarterly Journal of Microtcopical Science, 

 voL i. ; Pritchard, A Hittory of /nfutorial Animalctda, Living and 

 Fouil; Dujardin, Jliitoire Naturelle da Zoophytet Infutoira ; Hassall, 

 liritiik freik-Water Algae; Ehrenberg, Die Infunont-Thierchen; 

 Hooker, Briluk Flora, vol. ii. ; Agardh, Conipeclui Critical Diato- 

 mactarum; Papers in vols. i. & ii. of Quarterly Journal of ificroKOpical 

 Heience, by T. Brightwell, G. Shadbolt, Professor Gregory, Messrs. 

 Sollitt and Harrison; Papers in Annalt and Magazine of Natural 

 Hilary, by 3. Ralfs, Rev. W. Smith, G. H. K. Thwaite* ; Keporti of 

 SritiA Auociation ; Professor J. W. Bailey in Smithtonian Contri- 

 butioni to Knowledge.) 



DIC^E'LUS, a genus of Coleopterous Insect*. [Licisus.] 



DIC.iEUM. [CERTHIADJS; CIKSTBIDJB.] 



DI'CKRAS, a genus of Fossil Conchiferous Mollutca, allied in form 

 to Ckan.a. It includes species mostly occurring in the upper part of 

 the Oolitic strata and (D. Lontdalii) the lower part of the Greensand. 

 D'Orbigny ranks the Utter species as Caprotina. [CHAMACEA.] 



DICHKI.ASI'IS. [ClRBIPEDIA.] 



mCHBLEBTHIUM. [POKILOPODA.] 



DICHOBt'NK. [AHOPLOTHEBICM ; PACHTMBMATA.] 



IHCHOLOPHUa [CABIAMA.] 



DICHOTOMY, a term in BoUny employed to express a mode o; 

 branching amongst planU by constant forking. The first stem or vein 

 of a plant divides into two branches, each branch divides into two 

 others, and so on. It is only in the veins of fern leaves, and of those of 

 some of the specie* of Oontferte, and in the stem* of Ferns, that this 

 mode of division exist* a* a general character. It however doe* occur 

 ebewhere ; for example, the Doom-Palm (Crocifera. Thebaica) of Thebes 

 is remakable for its dichntomou* branches, and the Screw-Pines 

 (/'andanui) have a similar habit 



DICHROITE, a Mineral, also known by the names lolite, Peliome, 

 .ftnnkntilr, and Cordierile. It is an anhydrous silicate of alumina, 

 and occurs massive and crystallised. Its primary form is a right 

 rhombic prism, commonly crystallised in 6 or 1 2-sided prisms. The 

 cleavage parallel to the lateral plane*. It has a blue colour in 

 the direction of the axis, and yellowish-gray perpendicular to it ; 

 sometimes dull yellowish in both directions. The streak is white. 

 Fracture uneven and somewhat conchoidaL Its hardness is 7'0 to 7 '8. 

 Lustra vitreous. It is transparent and translucent. It* specific 

 gravity is 2'58. The massive varieties are amorphous. It* structure 

 U indistinctly ganular. It is found at Cabo de Gala in Spain, in 



Par the sbovt Illustrations we are Indebted to th Rev. W. Smith not only 

 far tkow pnblUbtd In vol. I. of hl Brlttah Dtatonwee**,' but through Mr. 

 I Wwt, la* srttot, for tevtnl from tlie unpublUhfd volume of tht 

 M* work. Mr. Wtt h al*o forolsbed some orifln.l drawing!. 



Greenland, at Bodenmais, in Bavaria, Norway, the United States, 4c. 

 The following is an analysii by Stromeyer : 



Silica 80-24 



Alumina 33'42 



Magnesia 10'84 



Protoxide of Iron 4-00 



Protoxide of Manganese .... 0'68 



Water 1'68 



DICLIDURUa [CHKIBOPTKBA.] 



DI'CLINOUS, an obsolete term in Botany, signifying that a plant 

 has its sexes in distinct flower*. It comprehend* all the Monoecious 

 and Dioecious plant* of Linmcus. 



DICOTYLE'DONS, a Natural Class of Plant*, deriving their name 

 from the embryo having in general two seed-leaves, or cotyledon* ; a 

 character t which there are however some exceptions. The genus 

 Ceratopkyllum has several : BertkoUetia appears to have none ; in 

 Trojxeolum and m/iny other* they are consolidated into one; and 

 Ciucuta is certainly destitute of them. Like all others therefore the 

 character derived from the cotyledons, nearly universal as it is, has 

 its exceptions. Hence botanists associate with the character derived 

 From the embryo others derived from the mode of growth, leaves, 

 flowers, &c., and the whole taken together give the real diagnosis of 

 the class to which the name of Dicotyledons is applied. 



The stems of Dicotyledons are probably in all cases branched, 

 except when a plant is from its weakness and minuteness unable to 

 reach any development beyond that of the first stage from the 

 plumule. In herbaceous plants that are called Htemless, a sort of 

 branching take* place by the formation of small short stems upon the 

 crown of the axis of growth. They have the pith, wood, and bark 

 distinctly separated, and the wood traversed by medullary rays. 

 This wood if more than one year old consists of concentric circles, 

 each of which is formed on the outside of that which immediately 



Ceded it ; the consequence of which is that the oldest part of a 

 ch will necessarily consist of the largest number of layers, and 

 will therefore be the thickest : hence the branches of Dicotyledons are 

 always cones, although usually very long ones [EiooKNs], and not 

 cylinders. But to some of these characters the exceptions are not a 

 few. It is difficult to trace any distinction of pith, wood, and bark in 

 the stem of the Water-Lily (Nympha-a) ; there are no concentric circles 

 in the wood of Arittolochta, and several other genera ; and it is impos- 

 sible to show by measurement that the stems of many irregular 

 tropical dicotyledonous climbers vary perceptibly in diameter for 

 considerable distances. 



The leaves of Dicotyledons are articulated with the stem, so that at 

 a particular time they are thrown off, and leave a clean scar behind 

 them, as in all the trees, whether deciduous or others, which are 

 found in the open air in England. Their veins are repeatedly 

 branched, so as to form a netted apparatus within the parenchyma. 

 But although by far the largest part of Dicotyledons are thus con- 

 structed, yet we have contradictions to this also. For instance, in 

 Dianthut, and a great many other genera, the leaves hav no veins 

 except the midrib, and there are many herbaceous plants whose leave* 

 never drop fairly off the stem*. 



The number of parts in the flower of dicotyledonous plants is four 

 or five ; that is, four or five sepals, four or five petals, and the same 

 number of stamens, present in either a complete or imperfect state ; 

 or if the number U greater it is some power of four or five. But in 

 Jtanuncului, Ficaria, most Anonaceous plants, and several more, the 

 parts in the flower are three. 



Finally, when the eeeds germinate the embryo simply extends the 

 point of its radicle in the form of a root to seek for nourishment in 

 the soil. [ExoRHiz.] But although this is, as far as is yet known, 

 a character without exception, nevertheless it has been too little 

 studied for us to assert that it i* more free from anomalies than other 



rhiUM.-S r-. 



Hence it is to be observed that it is neither the two cotyledons, 

 nor the exogenous stem, nor the concentric circles of wood, nor the 

 reticulated disarticulating leaves, nor the quinary or quaternary 

 flowers, nor the exorhizal germinations, which by themselveH charac- 

 terise the class of Dicotyledons, but the (.unl'inuti.iM uf those 

 characters ; and that the absence of some one of them is immaterial ; 

 so that a plant may be essentially a Dicotyledon, although it has any 

 number of cotyledons except two. 



The Dicotyledons of Jussieu comprehended the plants now sepa- 

 rated under the name of Gymnosperms, and together with Monoco- 

 tyledons and Acotyledons, constituted the whole vegetalle kingdom. 

 They probably consist of at least two-thirds of all known plant* : in 

 the state of trees, shrubs, or herbaceous plants, they are found 

 wherever phronogamio vegetation can exist, and they and Gymno- 

 sperms together constitute exclusively the arborescent scenery of all 

 cold countries. 



DICOTYLES. [Sure*.] 



DICRURUS. [LAKIADJL] 



DICTAMNUS, the botanical name of the fragrant horbaceous plant 

 called PrariwlU by gardeners. [FBAXINBLLA.J 



DICTY'OCHA, a genus of organised beings referred by Ehrenberg 

 to the ffariculacea or Diatomacea. They are characterised by an 



