36 One Thousand Objects 



The colour of this species is a pale brown. The ends 

 of the valves are obtuse, and its size is less than either of 

 the foregoing. It requires care to distinguish many of the 

 species in this genus from each other. The above species 

 of Pleurosigma are more or less common in all fresh-water 

 gatherings. (PI. IV., fig. 30.) 



MARINE DIATOMS, familiar to every microscopist, 

 are found all along the coast, parasitic on seaweeds, attached 

 to zoophytes, and often in great abundance in the stomachs 

 of marine animals, especially molluscs, and growing freely 

 in every tide pool. Collecting them is no very difficult 

 task; and, when unmixed with sand, boiling in nitric 

 acid, and afterwards free washings in distilled water, are 

 sufficient to procure the siliceous shields in a fit condition 

 for mounting as microscopical objects. The following 

 species are those most commonly met with in stale or 

 brackish water. 



257. VENTRICOSE EPITHEMIA (Epithemia ventricosa). 

 The valves in this genus are arcuate, or bent like a bow, 

 with transverse ribs. The form of the present species will 

 be better understood from the figure (PI. IV., fig. 36) than 

 any technical description. It occurs in brackish water. 



258. SIGMOID NITZSCHIA (Nitzsckta sigma). This is a 

 large genus of elongated form, with the valves linear or 

 line-like, and keeled with a punctate margin and tranverse 

 strise. In the front view the present species has the waved 

 form of a long S (/). Brackish water. 



259. BEAKED NITZSCHIA (Nitzschia vivax). The front 

 view in this species is straight, the valves bent like a bow 

 (arcuate), and beaked at the extremities, with distinct 

 striae terminating in marginal dots. Found in sand ripples. 



260. POINTED NITZSCHIA (Nitzschia dubid). In this 

 the valve is straight, with the ends tapering to a point. 

 The tranverse strias are faint. Common in all marine and 

 brackish-water gatherings, and resembles Tryblionella 

 acuminata, with which it is frequently associated. 



