40 One Thousand Objects 



with obtuse 'angles. Found in mud at the mouths of 

 rivers and in dredgings. (PI. IV., fig. 16.) 



284. ANGLE-BARRED TRICERATIUM (Triceratium alter- 

 nans). The sides are straight, with obtuse angles, which 

 latter are granulated, and separated by transverse lines from 

 the hexagonal centre. Found under similar conditions to 

 the foregoing. 



285. SHIELD COCCONEIS (Cocconeis scutellunt). The 

 valves in this genus are elliptical, with a line down the 

 middle, and a nodule in the centre. In this species 

 the valves are broadly elliptical, and the transverse lines 

 are concentric. It is said to occur in every quarter of the 

 globe, parasitic on the larger Algas. (PI. IV., fig. 18.) 



286. LONG-STEMMED ACHNANTHES (Achnanthes longi- 

 pes). In this genus the frustules grow attached to a 

 stem or stipes. The frustules are bent downwards, so 

 that the upper side is convex. It this species the stem is 

 long, and the valves ribbed. Parasitic on seaweeds. (PI. 



IV., fig. 22.) 



287. SHORT-STEMMED ACHNANTHES (Achnanthes bre- 

 vipcs). The stem is short, and the valves, though striate, 

 are not ribbed. It also grows attached to seaweeds. 



288. SAND AMPHORA (Amphora arenaria). In this 

 genus the valves are bent, with a nodule at the centre of 

 the inner edge. The frustules of the present species are 

 broadly linear, with rounded angles, and the valves are 

 very transparent. It occurs in sand ripples. 



289. OVAL AMPHORA (Amphora ovalis). The frustules 

 are oval, with broadly rounded ends, and the valves have 

 distinct beaded or moniliform strias. It is found in salt 

 or brackish water. (PI. IV., fig. 17.) 



290. ELLIPTIC NAVICULA (Navicula elliptica). This is 

 the largest genus of British diatoms. The form is simple, 

 more or less rectangular in the side view, with a longitu- 

 dinal clear space or line down the centre, a central and 

 terminal nodules. The present species is elliptical, with 

 rounded ends and distinct, beaded or moniliform striae. 



291. JENNER'S NAVICULA (Navicula Jenner'i). This 

 species has a waved central line, and the frustules are 



