208 DESCRIPTION OF [Polygastrica. 



moid- shaped lorica, devoid of striae ; when viewed edge- 

 ways, it is straight. (See group 146). It often contains 

 motile granules. The ova are of a golden-yellow ; colour- 

 less digestive cells are often visible. Found in fresh and 

 salt water. Size l-210th to l-140th. 



238. Navicula scalpum. The knife Navicula resembles 

 the preceding, but smaller. Found in sea-water ; and 

 fossil in the Schistus of Berlin. Length 1 -430th to 1 -290th. 



239. Navicula curvula. The crooked Navicula is 

 moveable, and has a narrow linear sublanceolate sigmoid 

 lorica, devoid of longitudinal striae or rays. Found 

 amongst oscillatoria. Size 1 -430th. 



240. Navicula arcus. The boiv Navicula has a narrow, 

 linear, curved, and smooth lorica, inflexed and umbilicated 

 at the centre. Found in mineral water. Length 1 -6000th 

 to l-570th. 



(6). Tramversely striated — internally ribbed Naviculee. 



SURIRELLA. 



241. Navicula sigmoidea is striated. Group 148 shews 

 to the left a specimen undergoing longitudinal self-division, 

 and the figure on the right is a side view. The relation of 

 the length to the number of stripes is as follows : — Those 

 individuals which were 1 -720th long had ten stripes; 

 1 -570th, fourteen; l-480th, fifteen; l-360th, twenty; 

 l-280th, twenty- seven ; l-140th, fifty-four; l-120th, 

 sixty; l-70th, one hundred and eight; l-48th, one 

 hundred and sixty; l-36th, two hundred and sixteen. 



242. Navicula vm^/is. The green Navicula is striated ; 

 has a straight lorica, truncated at the ends. Fig. 133 and 

 134 exhibit views of the shells empty, as found fossil; 



