Oct. 1, 1869.] 



HARDWICKE'S SCIENCE-GOSSIP. 



223 



Pleurosigma lanceolatum (fig. 1S5). — Valves small, 

 lancet-shaped, scarcely sigmoid ; median line more 

 so ; markings very fine, oblique. 



Pleurodynia Fasciola (fig. 180). — This species is 

 remarkable for its long narrow ends, which are 

 strongly curved in opposite directions ; body of the 

 valves lancet-shaped; markings dots, arranged in 

 squares, but requiring a high power to resolve them. 



Pleurosigma angulation (fig. 187). — Outline of valve 

 slightly angular at the centre; sigmoid curvature 

 moderate ; markings similar to the preceding. 



Toxonidea is a genus closely allied to Pleurosigma, 

 but the opposite sides of the valves are dissimilar, 

 one being convex and the other straight. 



! I'll ! 



Fig. J 87. 

 Pleurosigma angulutum. 



Fig. 188. 

 Toxonidea Gregoriana, x 400. 



Toxonidea Gregoriana (fig.188). — One side of valve 

 convex until it approaches the ends, where it curves 

 slightly outwards ; the other side straight ; but as it 

 approaches the ends it follows the curvature of the 

 opposite side ; median line same as convex margin ; 

 markings oblique, distinctly visible with a good 

 quarter-inch objective. 



Toxonidea insignis. — Smaller than the preceding 

 one; margin very convex; ends very slightly 

 curved; the other margin nearly straight the 

 entire length of the valve; median line convex, 

 much nearer the straight than the convex margin ; 

 markings much finer than in Toxonidea Gregoriana. 



The following species of the genus Nitzschia will 

 be found in the sand-gatherings. 



Nitzschia vivax (fig. 189). — Valve long, gradually 



tapering towards the pointed ends ; one margin with 

 a conspicuous row of dots, very slightly incurved at 

 the centre ; markings distinct transverse lines. 



Nitzschia virgata (fig. 190) resembles the pre- 

 ceding, but is shorter and more curved, and the 

 marginal dots are elongated into short bars ; mark- 

 ings transverse, very distinct. 



Fig. 189. 



« 



Fig. 190. 

 Nitzschia' 



Fig. 191. 

 Epithemia marina, x 4C0. 



Nitzschia vivax. virgata, x 400. a, valve; b, frustule 



Epithemia marina (fig. 191) resembles Nitzschia 

 vivax in outline, but wants the marginal dots : the 

 valve is crossed by distant lines, and is also dotted. 



Attheya decora (fig. 192).— This curious little form 

 is common in some gatherings : it must be searched 

 for before using the nitric acid, the separation of 

 the valves rendering it difficult to detect: the figure 

 shows the two valves connected by the zone. 



Fig. 192. Attheya decora, a, a, frustules ; A, valve. 



Asterionella Bleakleyii (fig. 193) cannot be found 

 after treatment with acid. Each ray of the star 

 consists of the two valves joined by the connecting 

 zone ; the frustules during life adhering to each 

 other by the enlarged ends. 



The circular species of Diatomaceae are not gene- 



