210 DESCRIPTION OF [Polygastrica. 



bury Fort; fossil at Santa Flora. Length 1 -430th to 

 l-120th. 



246. NAVICULA gibba. The gibbous Navicula is striated, 

 has a straight lorica, swelled at the middle, and gibbous. 

 Nine striae in l-100th of a line. Found, living, in brackish 

 water, at Gravesend, and fossil in the Isle of France. 

 Length l-430th to 1- 120th. 



247. NAVICULA (?) crux. The cross-like Navicula is 

 striated, has a short lorica, swelled very much laterally at 

 the middle, so as to appear like a cross. Seventeen striae 

 in 1- 100th of a line. Rare: found only fossil. Length 

 1-1 150th. 



248. NAVICULA (?) ylans. The acorn-like Navicula is 

 striated, has a short lorica, swelled at the centre in the 

 form of an acorn. Two or three striae in 1- 100th of a line. 

 Found only fossil. Length l-1150th to l-576th. 



249. NAVICULA capitata. The knobbed Navicula is 

 striated, has a short ovato-lanceolate lorica, extremities 

 constricted and obtuse. Ten transverse striae in l-100th 

 of a line. Length l-1150th to l-576th. 



250. NAVICULA. dicephala. The double-headed Navicula 

 is striated, has a bright lorica, constricted and obtuse at 

 both ends. Nineteen transverse striae in l-100th of a line. 

 Found fossil at Degernfort, in Sweden. Length 1- 860th 

 to l-480th. 



251. NAVICULA lanceolata. The lancet-shaped Navicula 

 is striated, has a lanceolate elongated lorica, gradually be- 

 coming acute at the ends. Thirteen transverse striae in 

 l-100th of a line. Length l-1150th to l-280th, 



252. NAVICULA (?) librile. The balance Navicula is 

 striated, has an elongated lorica, undulated at the sides, 



