Bacillaria.] THE INFUSORIA. 205 



found in the fossil condition ; and ten fossil species have 

 no living representative. They are found only in the 

 most recently-formed strata and tertiary formations ; 

 none have been found in the chalk. 



(a). Lorica smooth within ribless Naviculce. 



221. NAVICULA Phoenicenteron (Cymbella, Agardh.} 

 The reddish Navicula has a smooth siliceous lorica, of an 

 elongated lanceolate form, as shewn in the engraving, 

 plate iii., group 139 ; longitudinal striae are rare. The 

 central opening is oblong, its greatest diameter being 

 transverse. It is very motile. Found in ponds at Hamp- 

 stead; and fossil at Santa Fiora, in Tuscany. Length 

 1 -400th to l-140th. 



222. NAVICULA gracilis (Vibrio tripunct, M.) The 

 slender Navicula is smooth ; the central opening round, 

 and the ends truncated. It occurs abundantly with oscil- 

 latoria, and is found fossil at Cassel, &c. Length 1-1 500th 

 to 1-5 60th. 



223. NAVICULA ? pellucida. The furrowed Navicula has 

 a slender lorica, as shewn at fig. 140, which represents a 

 group of them ; and above is an outline section, taken 

 across the middle, to show the position of the central 

 furrow in each side; moveable. Found with the pre- 

 ceding. Length l-300th to 1- 140th. 



224. NAVICULA acus. The needle-shaped Navicula, 

 shewn at group 147, is smooth, straight, and pointed at 

 both ends. The ova grains are in the middle, and of a 

 yellow brown colour. This species resembles Closterium 

 setaceum, and some of the Fragilaria or Synedra, when 

 separated. Length l-430th to l-280th. 



