DIATOMACE^E. 



C 205 ] 



DICKIEIA. 



Frustulia (PL 14. fig. 17), Homfeocladia 

 (PL 14. fig. \5},Micromega(P\. 13. fig. 8), 

 Phlyct<enia,Rhaphidogl<Ba(P\.\4.fig.U). 

 Coh. 11. AMPHIPLEURE.E. Amphipleura 

 (PL 12. fig. 7), Berkeleyia (PL 14. tig. 8). 



Tribe II. Vitiates. Frustules with vittae 

 (in front view) ; valves not areolate. 



Subtribe. 1. Astomaticae. Valves without a 

 median nodule. 



Coh. 12. LICMOPHORE^. Licmophora 

 (PL IS. fig. 3), Podosphenia (PL 13. 

 fig. 17), Rhipidophora (PL 13. fig. 19). 

 Climacosphenia. 



Coh.lS.STRIATELLE^. &TMfeIfo (P1.13. 



fig. 20), Tessella (PL 14. fig. 11). 



Hyalosira (PL 13. fig. 1), Pleurodes- 

 mium, Rhabdonema (PL 13. fig. 18). 



Subtribe 2. Stomaticae. Valves with a median 



nodule. 



Coh. 14. TABELLARIE.E. Grammato- 

 phora (PL 12. fig.35), Tabellaria (PL 13. 

 fig. 27), Tetracyclus (PL 13. fig. 28). 



Anaulus, Biblarium, Stylobiblium, 



Terpsinoe (PL 19. fig. 10). 



Tribe III. Areolatce. Valves with cell-like 

 markings, often visible by direct (not oblique) 

 light. 



Subtribe. 1. Disciformes. Valves equal; 

 without appendages or processes. 



Coh. 15. COSCINODISCE^E. Actinocy- 

 clus (PL 18. fig. 43), Coscinodiscus 



(PL 18. fig. 32 ; PL 19. fig. 7). Ac- 



tinoptychus (PL 18. fig. 45), Arachnoi- 

 discus (PL 12. fig. 12), Asterolampra 

 (PL 19. fig. 5), Asteromphalos (PL 19. 

 fig.2),Halionyx,Heliopelta(Pl.l9.&gA), 

 Odontodiscus, Omphalopelta, Symbolo- 

 phora (PL 19. fig. 6), Systephania. 



Coh. 16. ANGULIFER.E. Amphitetras 



(PL 12. fig. 9). Amphipentas(P\. 19. 



fig. 11), Lithodesmium (PL 13. fig. 4). 



Subtribe. 2. Appendiculatse. Valves with 



processes or appendages. 

 Coh. 17. EUPODISCE.E. Eupodiscus 



(PL 12. figs. 30, 31). 

 Coh. 18. BIDDULPHI.E. Biddulphia 



(PL 12. fig. 15; PL 14. fig. 9), Isthmia, 



(PL 13. fig. 2). Chatoceras, Hemi- 



aulus (PL 19. fig. 3), ZygocerostPl. 14. 



fig. 13). 

 Coh. 19. ANGULAT^E. Triceratium 



(PL 13. fig. 29). Syndendrium. 



Coh. 20. ACTINISCI^E. Actiniscus, 



Mesocena (PL 19. fig. 1). 



BIBL. Ralfs, Ann. Nat. Hist. 1843. xi. 

 p. 447, xii. pp. 104. 2/1. 346. 457; Thwaites, 

 Ann. Nat. Hist. 1847. xix. p. 200, xx. pp. 9. 

 343. 1848. 2nd ser. p. 161 ; Dickie, 1848, 

 2nd ser. ii. pp. 93. 153; Smith, Brit. Diatom.; 

 id. Ann. N. H. 1850. 2nd ser. v. p. 121, 

 Microscopic Journal, part 10. 1854 ; Bailey, 

 Silliman's Journ. xli. xlii. ; id. Ann. N. H. 

 1851. viii. p. 157; Ehrenberg, Infusionsthier- 

 chen; id. Abhandl. d. Berl. Akad. 1839. 

 1840; id. Bericht d. Berl. Akad. passim; 

 id. Mikrogeologie ; Brebisson, Considera- 

 tions sur les Diatomees; Rabenhorst, Die 

 Siisswasser Diatom. ; Kiitzing, Bacillar. and 

 Species Algarum ; Pritchard, Infusorial Ani- 

 malcules; Alex. Braun, Verjungung (Ray 

 Soc. Vol. 1853); Nageli, Einzell. Algen. p. 9; 

 Focke, Physiologisch. Studien, Heft ii. 1853; 

 Meneghini, Suit. Animalit. &c. (Ray Soc. 

 Fo/.1853); Gregory, Mic. Journ.pt. 10. 1854. 



DICELLA, Werneck. A genus of Infu- 

 soria, of the family Trachelina (?), Ehr. 



Char. Those of Bursaria, with two im- 

 moveable setaceous appendages. 



B. appendieulata. 



DICHROISM (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 different 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 chro- 

 mium and ammonia. 



Dichroism depends upon the absorption of 

 some of the coloured rays of the polarized 

 light in the passage through the crystal, this 

 absorption varying with the different relative 

 positions of the planes of primitive polariza- 

 tion of these rays, to the axis of double 

 refraction of the crystals, so that the two 

 pencils formed by double refraction are 

 differently 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 potash they 

 are blue and green, and hi the tourmaline 

 they are not always the same. The variation 

 in colour is entirely independent of the 

 thickness of the crystal. 



BIBL. Brewster, Phil. Trans. 1819, and 

 Optics, p. 353; Herschel, Encyc. Metrop. 

 art. Light, p. 1064. 



DICKIEIA, Berkeley and Ralfs. A genus 

 of Diatomaceae. 



