DIATOMELLA. 



[ 258 ] 



DICLADIA. 



1 (figs.), f'br. in Qx. M. Jn. 18G3, iii. 120; 

 Antelmiiielli, Qn. M. Jn. 1868, 254 {repro- 

 duct.) ; Flfigel, Arch. mikr. An. iv. 472 ; 

 Macdonald, Ann. N. H. 18G9, iii. 1 ; INIa- 

 noury, I)iat. 1870; Fritscli & Miiller, 

 Slailpfur, 1870 {ejcellent photof/raphfi) ; 

 Donkiu, Br. Diaf. 1871 ; Rabenliorst, Fl. 

 Alff. i. ; Pfitzer, Hanstein's Bot. Ahh. 1871, 

 ii. 120; Millarclet and Kraus, Compt. 

 rendits, Ixvi. 505 ; Aslieusky, Bot. Zcif. 

 1809, 790; Luders, Bot. Zeit. 1 862 (colourimj 

 matter) ; Hickie, Schumann''s Diat. formulce, 

 M. M. J. 1875, xiv, 6 ; Habirshaw, Qdal. 

 Bint. 1877 ; Lewis, Diat. of United-St. Sea- 

 board; Schmidt, Atlas {pqs. of all species 

 8,-c.), 1878; Pettitt, Jn." Mic.'Soc. 1878, i. 

 237, pis. (Campbell Island ), & Bidl. Soc. 

 Bot. d. France, xxiii. & xxiv. (Kitton, M. 

 M. Jn. 1877,xviii. 10 & 05; classn.); Cox, 

 Amer. Jn. 3Iic. 1878, iii. 100 ; lieurck, 

 Syn. mat. 1880 (Bekjiqne, pis.) ; Cleve & 

 Grunow, Diat. Arct. 1880; Laiizi, A71. 

 Soc. Beh/e (Jn. Mic. Soc. ii. 1879, 38, 

 </(«//?<«);' Mereschkowsky, Bot. Zeit. 1880, 

 xxxYiii. 529 (Jn. Mic. Soc. 1881, i. 102) ; 

 Borscow, Bac. Busslands, 1873 ; Eng-el- 

 mann, Bot. Zeit. 1879; Ilallier, Diat. 1880 

 (movement). 



DIATOxMELLA, Grev. D. Balfoririana, 

 Qy.-=Grammat(iphora B.j Smith (PI. 51. 

 fig. IG) :=Disiphimia aastralis, Ehr. 



DIOEL'LA, AVeineck.— A doubtful ge- 

 nus of Infusoria (Berl. Ber. 1841, 377). 



DICIIONE'MA, N. ab Es.— A genus of 

 Ilymenomycetous Euugi, iuhabitiug tro- 

 pical America and I'olyuesia. 



D. sericeinn consists of a reticulation of 

 Confervoid filaments, interlaced "with myce- 

 lium, and at length, like Cora, producing a 

 distinct hymenium which cracks into 

 scattered patches. This genus is some- 

 times called Dictyoncma. Spreading in an 

 orbicular form, from branches of trees; a 

 curious microscopic object (,Leightou). 

 BiBL. Esenb. JS. Acta, xiii. 12. 

 DI'CHROISM (double colour) is the term 

 applied to the property possessed by many 

 doubly refracting crystalline substances, of 

 exhibiting two colours when light is trans- 

 mitted through them in difierent positions. 

 It may be observed under the microscope in 

 crystals of the tourmaline, the acetate of 

 copper, the chloride of palladium, and the 

 oxalate of chromium and potash, or of 

 chromium and ammonia. 



Dichioism depends upon the absorption of 

 some of the coloured lays of the polarized 

 light in their passage tlirough the crystal — 



this absorption varying with the different 

 relative positions of the planes of primitive 

 polarization of these rays to the axis of 

 double refraction of the crystals, so that the 

 two pencils formed by double refraction are 

 dillerently coloured. 



In the acetate of copper, the two colours 

 are deep blue and yellowish green ; in the 

 chloride of palladium, they are red and 

 green ; in the oxalate of chromium and 

 Ijotasli they are blue aud green, and in the 

 tourmaline they are not always the same; 

 The variation in colour is entireh^ indepen- 

 dent of the thickness of the crj'stal. 



BiBL. Brewster, Pldl. Tr. 1819, and 

 Optics, 353 ; Ilerschel, Fncyc. Metr. art. 

 Liqht, 1064. 



biCKIEI'A, Berkeleyand Ealfs.— A ge- 

 nus of Diatomaceae. 



Char. Erustules navicular, irregularly 

 scattered through a flat undulate frond or 

 subgelatinous layer, narrowed at the base so 

 as to appear substipitate. 



D. nhoides (PI. 19. fig. 16: a, frond, 

 nat. size ; b, portion, magnified ; c, prepared 

 frustule, front view ; d, valve). Stipes very 

 short, capillary ; frond oblong, irregularly 

 lobed or crenulate; frustules oblong, ob- 

 tuse-angled, truncate at the ends: valves 

 oblong, with a stauros ; length of frond 

 1 to U" ; of frustules, 1-1000 to 1-720" ; 

 marine. 



Frond very pale purplish white. Recent 

 frustules with a round colourless spot at 

 each of the four angles (in the front view). 

 Found in shallow pools between higb aud 

 low-water mark. 



D. piiinata. Frond irregularly divided, 

 or laciniate ; v. like Naricida. Marine. 



BiBL. Berkeley and Ralls, Ann. JV. H. 

 1844, xiv. 238, and 1851, viii. 204 ; Kiit- 

 zing, Bacill. and Sp. Alq. 109; Thwaites, 

 Ann. N. II. 1848, i. 171 ;" Smith, Brit. Diat. 

 ii. 166. 



DICKSO'NIA, L'lleritier.— A genus of 

 Dicksonieaj (Polvpodiaceous Ferns), in- 

 cluding fine arborescent species. All exotic. 



DlCKSO'NIEy^E.— A family of Polypo- 

 diaceous Ferns. 



Genera : Onoclea, Hypoderris, Woodsia, 

 Spheeropteris, Dicksonia, nnd Deparia, 



BiBL. Hooker, Syn. Fil. 45. 



DICLA'DIA, Ehr.— A genus of Diato- 

 macefc. 



Char. Frustules single ; valves imequal, 

 one turgid aud simplej the other two- 

 hovued, the horns sometimes branched. 

 Marine and fossil. 



