MANN DIATOMS OF CHE ALBATROSS VOYAGES. 231 



A. Valve circular, its surface flal or convex, sometimes hemispherical; 



diameter generally shorter than valval (excepl in a fen species oi Melosira 

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



I OIDE \l. 



a. Valves circular; no1 divided into definite sectors by ridges or partiti 

 though sometimes having radial Lines of interrupted dashes. Cosci 

 dis< i \ i . 

 a. Frustules usually united into chains; each frustule in zonal view 

 short-cylindrical, spherical, or nearly Lenticular; zone sculptured. 

 (Melosirin \l 

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

 form and delicate or with a hyaline or rugose central area 

 surrounded by a strongly sculptured band; qo true central 



umbilicus Lfi lot 



Chains usually short, of no1 more than 2 to l frustules; 



never having a strongly sculptured external pari or band; a 

 distinct central umbilicus, granular, usually contrasting 

 strongly with the resl of tin- valve and separated from ii by 

 an evidenl wavy ridge or lino; diameter of the umbilicus 

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

 extreme oases disappearing; marking of the resl of 'ho valve 

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

 ing pa Horn: sometimes divided into obscure sectors by radial 



delicate Lines / 



Chain- of many or few frustules united to each other by spines; 

 zonal diameter of each frustule equaling or much exceeding 

 the valval; qo central umbilicus or hyaline area; valve hav- 

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

 marginal Bpines, i rarely near the center or scattered, I uniting 



the frustules into a chain Stephanopyi is p. 2 



6. Frustules solitary: zone narrow, hyaline, or in rare cases obscurely 

 dotted; valve circular, surface slightly convex, rarely strongly 

 convex, sometimes approaching flat or barely concave. 



i !( >s< [NODIS< I N 



Valve uniformly marked, with a hexagonal network or with 

 beading arranged in radial, concentric, or decussating order; 

 no centra] area sharply separated from the outer portion and 

 strikingly contrasted with it by difference in character or 

 coarseness of marking; marginal spines wanting or minute. 

 Valve withoul pseudonodule near or at the margin. 



Co> - p. 246). 



Valve with a pseudonodule near or at the margin. 



Valve with two strongly contrasted areas of different markings; 

 a large clearly separated, slightly depressed central 

 marked with network or beading, and a broad encircling . 

 or band marked with network of different sized mesh from 

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

 spines ( 'raspedodiscus p. 2 



Valve with two strongly contrasted areas; a central area hyaline 

 or lightly marked with radiating dotted linos, and a broad or 

 narrow encircling area or hand strongly marked with radial 

 rtea or Lines of beading and usually bearing a row of submar- 

 ginal spines; a portion ol the central area is concentrically or 

 excentrically elevated, so thai the valve profile, as Been in 



zonal view, appears undulate I 



31713— vol 10, pt 5—07 '1 



