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MANX — DIATOMS OF THE ALBATROSS VOYAGES. 231 



A. Valv(^ circular, its surface flat or convex, som(*timcs heinispliorical; zonal 



diameter generally shorter than valval (except in a few species of Melosira 

 and Stephanopyxis); spines frequent; horns, when present, small. (Dis- 



COIDEAK.) 



a. Valves circular; not di^ ided into defmite sectors by ridges or partitions, 

 th(jugh sometimes ha\ing radial lines of interrupted dashes. (Coscino- 



ni8CEAE,) 



a, Frustules usually united into chains; each frustnle in zonal view 

 ehort-cylinth'ical, spht^rical, or nearly lenticular; zone sculptured, 



(MELOSIiaNAE.) 



Chains usually of many frustules; marking of valve either uni. 

 form and delicate or with a h}-alinc or rugose central area 

 surrounded by a strongly sculptured band; no true centi'al 



umbilicus Melosira (p. 236). 



Chains usually short, of not more than 2 to 4 frustules; valve 

 never liaviiig a strongly sculptured external part or band; a 

 distinct central umbilicus, granular, usually contrasting 

 strongly with the rest of the valve and separated from it by 

 an evident wavy ridge or line; diameter of tlie umbilicus 

 varying from one-half that of the valve to very minute, in 

 extreme cases disappearing; marking of the rest of the valve 

 delicate, radially arranged, generally of the watch-case mill- 

 ing pattern; sometimes divid<'d into obscure sectors by radial 



delicate lines Podosira (p. 240), 



Chains of many or few frustules imited to each other by spines; 

 zonal iliameter of each frustule equaling or much exceeding 

 the valval; no central uml>ilicuR or h valine area; valve hav- 

 ing generally one? or more circles of strong marginal or sub- 

 marginal sj)im's, (rarely near the center or scattered,) uniting 



the frustules into a chain Stephanopyxis (p. 243). 



b. Frustules solitary; zone narrow, hyaline, or in rare cases obscurely 

 dotted; valve cii'culur, surface t^lighlly convex, rarely strongly 

 convex, sonn^tinies approaching flat or barely concave. 



(COSCINODISCINAE.) 



Valve uniformly marked, with a hexagonal network or with 

 beading arranged in radial, (oncentric, or decussating order; 

 no central area sharj)]y separated from the outer portion and 

 strikingly (^ontrastenl with il l)y difference in character or 

 coarseness of marking; niargiruil s])ines wanting or minute. 

 Valve without pseudonodule near or at the margin. 



Coscinodiscas (p. 246), 

 Valve with a pseudonodule near or at tlie margin. 



Actinocydus (p, 2G0). 



Valve with two strongly contrasted areas of different markings; 

 a large clearly separated, slightly depressed central area 

 marked with uv^work or beading, and a broad encircling area 

 or band mark( d with network of different sized mesh from 

 that of the central area; margin of valve massive; no marginal 

 spines Cras pedodiscus (p. 2(54). 



Valve with two strongly contrasted areas; a central area hyaline 

 or lightly marked with radiating dotted HneSj and a broad or 

 narrow^ encircling area or band strongly marked witli radial 

 cost(^a or lines of beading and usually bearing a row of submar- 

 ginal spiiies; a portion of the central area is concentrically or 

 excentrically elevated, so that the valve profile, as seen in 

 zonal vi(*w, appears undtilate Cyclotella (p. 265). 



31713— VOL 10, PT o— 07 2 



