222 



IHARDWICKE'S SCIENCE-GOSSIP. 



[Oct. 1, 1869. 



which the valves frequently occur in situ after boiling 

 iu acid : they adhere to the connecting zone more 

 strongly than many other species. The outline of the 

 frustule is oblong, sometimes nearly square ; the ends 

 straight or slightly rounded ; the sides more or less 

 constricted at the centre; the valve very narrow 

 and straight, and the extremes rounded ; the mark- 

 ings short and far apart. 



Fig. 179- 



Navicula retusa, x 400. 



a, valve ; b, frustule. 



Fig. 180. 

 Navicula cruciformis, x 400. 



Navicula cruciformis (fig. 180). — A curious little 

 form with straight sides, slightly tapering as they 

 approach the rounded ends ; the median line is 

 sometimes wavy ; the markings are slightly radiant 

 lines, reaching the central one, but are entirely 

 absent opposite the central nodule, producing a 

 blank space across the valve. 



Navicula Trevelyana (fig. 181) has the sides 

 straight, the ends rounded; markings coarse and 

 radiant, suddenly shortening as they approach the 

 centre, leaving a large circular blank space round 

 the central nodule ; median line conspicuously 

 undulated. 



Fig. 181. 



Fig. 182. 



Ifameula Trevelyana, x 400. Navicula Clepsydra, x 400. 



Navicula Clepsydra (fig. 182). — A very handsome 

 species ; the valve long, and gradually tapering to 

 the rounded ends : markings consist of irregular- 



shaped dots, arranged in transverse lines : au oval 

 smooth space surrounds the nodule. 



Amphora.— The species of this genus somewhat 

 resemble Navicula ; the valves when separated have 

 one margin convex and the other straight. Several 

 species of this genus occur in sand-gathering. The 

 following I have found most frequent. 



Amphora arenaria (fig. 183). — This species is easily 

 recognized by its transparency and apparent absence 

 of markings ; but with a high power it is found to 

 possess them : neither the median line nor nodule is 

 central, the former being curved in an opposite 

 direction to the convex margin of the valve. 



a b 



Fig. 183. Amphora arenaria, x 400. 



a, a, valves ; b, frustule. 



The observer will scarcely fail to detect in his 

 gathering some representatives of the genus Pleuro- 

 sigma : they are distinguished by the valves being 

 more or less curved in opposite directions; the 

 median line also following the contour of the 

 valves. 



Pleuros'ujma asttiarii (fig. 184). — A small form, 

 somewhat broad at the centre ; the outline of the valve 

 is less sigmoid (S-like) than the median line ; the 



Fig. 184. 



Pleurosigma 



cestuarii. 



Fig. 185. 



Pleurosigma 



lanceolatum, x 400. 



Fig. 186. 



Pleurosigma 



Faaciola. 



markings consist of minute dots, arranged in oblique 

 lines, requiring a good quarter-inch object-glass and 

 oblique light to resolve them. 



