206 DESCRIPTION OF [Polygastrica. 



225. Navicula umbonata. The knot-hearing Navicida 

 is straight and smooth ; the side is constricted near the 

 middle, and hence the ends appear enlarged, from which 

 it derives its specific name. Found both in salt and fresh 

 water; also fossil. Length l-430th to l-240th. 



226. Navicula fulva. The yellow Navicula has a 

 smooth, broad, lanceolate lorica ; near the ends, the shell 

 is slightly produced in the form of a rostrum. The ova 

 are yellowish, brown, or green ; the medial aperture 

 round. Found with oscillatoria in turbid water ; also 

 voung specimens, fossil, in Bohemia. Length 1-1 150th to 

 1-1 80th. 



227. Navicula amphisbaena has a smooth lorica of the 

 form shewn at group 141 ; the right hand figure is a side view. 

 The central opening is round, and the ova are of a golden - 

 yellow colour. Ehrenberg remarks, that the vibratile 

 process seen by Bory St. Vincent was in fact the locomo- 

 tive organ, the action of which is readily seen, though it 

 protrudes but a very little beyond the lorica. In the 

 specimens figured, transverse self division of the ova has 

 commenced, and two small gland-like spots are visible. 

 Length 1-1 700th to l-240th. 



228. 'NA.vicuL.Ajjlatistoma. The broad-mouthed Navi- 

 cula has a smooth lorica of the form shewn at fig. 142. 

 The central opening is lengthened transversely. Found 

 with oscillatoria. Length 1-1 100th to l-240th. 



229. Navicula tiodosa. The nodose Navicula is 

 smooth, has a linear lorica, with three lateral undulations, 

 or knots, at the middle, and the extremities constricted 

 into the form of an obtuse beak, as seen at fig. 143 ; cen- 

 tral opening round. Length 1 -430th. 



