410 DESCRIPTION o¥ [Pol^ffasfrica. 



opening small, round. Length l-70th. Found in phosphorescent 

 sea-water. 



Nayicula hippocampus. — Smooth, sigmoid, lanceolate; apices obtuse ; 

 laterally linear and truncate (gi'oup 145.) Sometimes it has deli- 

 cate longitudinal striae. Length l-90th to l-70th. In fresh and 

 salt water. This shell is the well-known test for microscopes. See 

 descriptions of Plates 19 and 20. 



N. Sipna = N. acuminata (Kiitz.) — Smooth, lanceolate, sigmoid, 

 ends attenuate, obtuse; laterally straight (group 146.) It often 

 contains motile granules, and has a golden-yellow colour. Length 

 1-2 10th to 1-1 40th. In fresh and salt water. 



N. scalprum. — Smaller, slightly sigmoid ; apices gradually taper- 

 ing, obtuse, with longitudinal lines. Length 1 -430th to 1 -290th. 



N. curviila. — Narrow, linear lanceolate, sigmoid, apices rather 

 obtuse ; no longitudinal striae. Lateral view straight. Fresh water. 



N. Trochus. — Short, smooth, enlarged at the centre, constricted 

 near the ends, which are much produced and truncate. The surface 

 is marked by several longitudinal stripes. Length 1 -860th. Fossil 

 in Sweden. 



N. AgeUus. — Large, of a sigmoid lanceolate figure on the back, 

 with very fine longitudinal lines, and having a furrowed space ; 

 straight and nearly linear on the side, with subacute extremities. 

 Length l-180th It is more slender and longer than N. Hippo- 

 eampus. In fresh water. Common. 



N. linodis. — Probably but the young of N. hihrile. Fossil at San 

 Fiore, Italy == Fragilaria linodis (Ehr.) 1843. 



N. carinata. — Lanceolate, linear laterally, and with a broad longi- 

 tudinal dorsal keel. Length 1-2 16th. Fossil on shores of the 

 Rhine in volcanic schists. 



N. eurysoma. — Small, ovate; ends obtuse; quite smooth; margin 

 distinct. Found in African chalk-marl. Not improbably a Cocconeis. 



N. inversa. — Short, dorsally narrow and sigmoid, "with subacute 

 ends ; laterally quadrangular, very broad, constricted at the centre, 

 with the ends widely truncate, and marginal glands (?) It moves 

 quickly; is allied to N. alata, but wants the winged portions. 

 Length 1-5 76th. 



