TERMINOLOGY AND CLASSIFICATION OF DIATOMS. 87 



capitate (fig. 23 D.), as in Navicula rhynchocephala (pi. 3, f. 119), or rostrate- 

 capitate (fig. 23, E.), as in Navicula mesolepta (pi. 2, fig. 96). 



The valve shows a lumen when its apex is more brilliant than the 

 other portions, which results from a thickening of the internal terminal 

 margin of the valve, such deposit of silica refracting the light with 

 greater power. Ex. Stauroneis acuta (pi. 1, f. 51). 



4. Processes. Processes are projections which are generally seen to be 

 standing out beyond the other parts of a diatom when it is examined in the 

 girdle view; they are either spines, as in Chceioceros (pi. 18, f. 604) or 

 places of attachment in filamentous (and probably other) forms, as in 

 Biddulphia (pi. 20, f. 630-636). 



The term process is hardly applicable to the simple blank spaces 

 that are seen on the concave valve of Gcphyria, or on the valves of 

 Dimeregramma or Plagiogramma. 



Alee or wings are expansions usually of the margins of the valves, 

 as in the Surirella (pi. 13); at other times they arise on the surface 

 of the valve itself, as in the genus Amphiprora (pi. 5, f. 287-29-?). When 

 the aim are only slightly projecting, as in the Nitzschia (pi. 15), the 

 valve is said to be Carinate (like a keel). 



There must also be classed with processes the Ocelli which are pseudo- 

 openings (in reality true processes) which present well marked external 

 outlines when the valve is seen in the girdle view, as in the genus Auliscus 

 (pi. 21, f. 646). 



B. Microscopical Structure of the Valve. 



The structure of the diatom valve has formed the subject of 

 numerous and important works, among which must be specially 

 mentioned those of Messrs. Flogel ('), Otto M idler ( 2 ), Prinz and Van 

 Ermengem ( 3 ), Cox ( 4 ), Haughton-Gill ( 5 ), and Deby ( 6 ). Flogel has 



( 1 ) Flogel : Untersuchungen iiber die Structur der Zellwand in der Gattung Pleurosigma. 

 Archiv, f. Mik. Anat. VI., 1870, pages 472-514. 



( 2 ) Uber den feineren Bail der Zellwand der Bacillariaceen insbesondere der Triceratium und 

 der P/eurosigma. Reiehert und Du Bois-Raymond's Archiv, fur Physiol., 1871. 



(3) Recherches sur la structure de quelques Diatom^es contenues dans le " Cemenstein " du 

 Jutland, par. MM. W. Prinz et E. Van Ermengem, Annales de la Soc. Beige de Microscopie t. viii. 



(4) Structure of the Diatom Shell. By Jacob D. Cox. Amer. Month. Micr. Journal, March, 

 April, May, June, 1884. This is the most complete general study on diatom valves up to the 

 present time. The author, in support cf his statements, has made a series of photographs of 

 broken valves, a collection of which he has been good enough to send us. In several points, the 

 ideas of Mr. Cox closely correspond with our own. 



( s ) On some methods of preparing Diatoms so as to exhibit clearly the nature of their markings, 

 by C. Haughton Gill, J.R.M.S., 1890, pp. 425-8. 



On the Structure of certain Diatom-valves as shewn by sections of charged specimens, by 

 C. Haughton Gill, J.R.M.S., 1891, pp. 441-2. 



( 6 ) On the Microscopical Structure of the Diatom valve, by Julien Deby, in Journal of the 

 Queckett Mic. Club, Sept., 1886. 



Id. Sur la structure Microscopique des valves des Diatom^es par Julien Deby, avec une 

 planche, Journal de Micrographie, 1886. 



