Bacillaria.] THE INFUSORIA. 209 



Jiff. 135 and 136, recent species, the arrows indicating the 

 direction of the current produced. This species has fifteen 

 striae {cellules) internally, in every hundredth part of a 

 line of its length. In the interior, numerous changeable 

 vesicles are seen, connected together by means of an 

 irritable gelatinous matter, which is as clear as crystal, and 

 from whose motion these stomach -cells often appear to 

 tremble. Ehrenberg has noticed moveable dark spots 

 near the extremity of some specimens, similar to what is 

 seen in Closterium, &c. The progress of longitudinal self- 

 division may often be observed beneath the siliceous lorica. 

 The six openings of the lorica are easily seen, three being 

 upon the upper surface and three on the lower. The 

 lorica near the central opening being depressed, the 

 aperture appears eccentric, in respect to the medial line. 

 Found at Hampstead, and fossil in Bohemia, Sweden, &c. 

 Length l-1150th to l-70th. 



243. Navicula macilenta is striated, has a straight 

 slender lorica, truncated on one side, and rounded on the 

 other. Twenty-three transverse striae are seen in 1- 100th 

 of a line. Found fossil. Size l-140th. 



244. Navicula viridula. The greenish Navicula has a 

 straight lanceolate lorica, slender and truncated at one 

 side, and attenuated and obtuse on the other. Thirteen 

 or fifteen striae are seen in 1-lOOthof a line. Found living 

 and fossil. Size l-3000th to 1 -280th. 



245. Navicula inequalis. The unequal Navicula is 

 striated and unequally convex. (See group 154.) The 

 clusters of ova are of a yellowish colour. In 1-lOOth of 

 a line are ten or eleven striae. This species forms the 

 passage to the genus Eunotia. Found, living, at Til- 



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