408 DESCRIPTION OF \^Polygasirica, 



SuETKELLA ovttta. — Small, laterally -widely cuneate and truncate ; 

 dorsally ovate, with delicate marginal striae, seven to nine in 

 l-1200tli. 



Var. (h.) dorsally equally elliptic. Length 1-I200thto l-560th. 

 Common in ponds. 



S. (?) ornata. — Elongate, truncate at each end, with obtuse angles ; 

 longitudinally cleft, and ornamented with minute puncta, disposed 

 in decussating lines. Length l-280th. Breadth l-960th. Among 

 Algae, Genoa. Kiitzing has seen this form but once. 



S. (?) Adriatica = Podocystis Adn'atica. Small, supported on a 

 short stipes, cuneate laterally ; dorsally obvate ; striae transverse, 

 eleven to twelve in l-1200th. On Callithamnion, at Trieste. 

 Length 1 -620th. 



S. minuta, (Smith.) — Frustule on front view (laterally Ehr.) wedge- 

 shaped ; on side view (dorsally, Ehr. ) elliptical or slightly ovate, 

 with ends more or less rounded ; costae marginal. Average length 

 l-1200th. Greatest breadth l-2500th. In streams. 



S. salina (Smith.) — Erustule on front view wedge-shaped; on side 

 view ovate ; the larger end rounded, and the smaller more or less 

 pointed; costae marginal. Average length 1-6 00 th; greatest breadth 

 1-1 200th. In salt-water ditches, Poole Bay. 



" This nearly resembles S. mimita, but is a salt-water species, 

 usually larger and distinctly ovate, and with the smaller extremity 

 of the valve, in most of the frustules, somewhat attenuated. On 

 the other hand, it is much smaller than S. oralis (Kiitz.) less oblong 

 and stout, and of marine habitat. 



S. circumsuta (Bailey.) — Outline nearly elliptical, with a scarcely 

 perceptible constriction at the middle ; surface with very minute 

 granulations, and a faint longitudinal line through the middle ; 

 edges with a continuous row of nearly obsolete pinnulae. Hudson 

 Eiver, west points of Florida. 



Sub-genus I^avictjla — True Navicula. — Lorica smooth, or with lon- 

 gitudinal lines or stripes ; central aperture round. This last circum- 

 stance distinguishes it from Stauroneis, whilst its general symmetry 

 separates it from the smooth forms of Cocconeis, which has one 

 surface depressed or flattened. 



