52 GEOLOGY OF THE FIRST DISTRICT. 
aa. Serrated,. Arthrodesraus * 
bb. Not serrated,. Odontella* 
2. In the form of discs or plates. 
aa. Several in each disc,. Micrasterias.* 
bb. Two in each disc,. Euastrum.* 
cc. Isolated in plates,. Microtheca. 
b. Two or more valves,.. Naviculacea. 
a. Round. 
1. Globular,.. Pyxidicula.* 
2. Forming polypidoms. 
aa. One cell, chain filiform,. Gaillonella* 
bb. Several concentric cells,. Actinocyclus.* 
b. Prismatic. 
1, Division perfect, never forming bands. 
aa. With six (?) openings,. Navicula.* 
bb. With four openings, . Eunotia.* 
cc. With one opening,. Cocconeis. 
2. Division imperfeet, forming bands, 
aa. Articulated. 
aa. Prisms (“baguettes”),. Bacillaria.* 
bb. Plates,. Tessella.* 
bb. Without articulations. 
ffl®. Strait ribbands,. Fragillaria.* 
bb. Spiral ribbands,. Meridion.* 
B. Fixed, . Echinella. 
a. Wider than long,. Isthmia.* 
b. Longer than wide. 
a. Without a pedicel. 
1. Prismatic,. Synedra.* 
2. Wedgeform,. Podosphenia.* 
b. Supported by a pedicel. 
1. Wedgeform. 
aa. Dichotomous,. Gomphonema.* 
bb. Verticillate,. Echinella.* 
2. Fixed perpendicularly,. Cocconema.* 
3. Fixed obliquely. 
aa. Opening in the middle,.... Achnanthes.* 
bb. Without opening in the middle,. Striatella.* 
B. Carapace double, . Lacernata. 
A. Surrounded by an amorphous gelatinous mass. 
a. Scattered,.. Frustulia. 
b. Joined in rings,... Syncyclia. 
B. Surrounded by membranous tubes. 
