440 BESCEIPTION OF IPolygastrica. 



Genus Sphexosiea (Ehr.) — Has the general cliaracters of Fragilaria, 

 but is made up of wedge-shaped bacilli, and so approaches Gompho- 

 nema. It occurs as a flattened band ; the central umbilicus of the 

 segments is distinct. 



S. Catena. — Segments smooth, with the general form of Gom- 

 pTionema Augims ; dorsally with one apex mucronate, the other 

 gradually tapering and slightly obtuse. In fresh water. Mexico. 

 (P. 14, f. 30). 



Genus SxAXJROsmA (Ehr.) — The form of this genus is that of quad- 

 rangular Fragilaria : it is distinguished from the larger forms of the 

 allied genus Ampkitetras, by the absence of openings at the four angles. 



S. construens. — Very small; smooth, spindle-shaped, angles pro- 

 duced, not quite equal. Length 1 -6000th (P. 24, f. 5). 



S. amphilepta. — Very small ; smooth, produced at the angles ; two 

 opposite angles larger and more slender than the others. 



S. pinnata. — -Very small, but longer than the preceding two, pro- 

 duced at the angles, and on each side more so than the other. 



S. trigongyla. — Characters unknown. 



Genus Sxephanodiscus (Ehr.) — Lorica of two equal orbicular valves, 

 not in chains, without apertures or internal septa; valves equal, 

 radiated, but with no constant number of rays; not cellular, but 

 characterized by a marginal crown of denticles on each side. 



Steplianodisci approximate in character to Biscoplece, but differ 

 from them by the circlet of hooks. They recal the general form of 

 Gallionellce, but do not occur in chains. 



S. Berolinensis ; Small ; nummiform (money-shaped), surface level, 

 with acute marginal denticles (often thirty-two) on each side, with 

 brown lobed ovaries (viz., the internal granular substance) ; disc 

 minutely radiated. Diameter l-1152nd. Alive at Berlin. 



S. NiagarcB. — Larger, plane, nummiform, with acute marginal 

 (often sixty-four) denticles on each side ; the disc oftenwith sixty- 

 four granular rays ; granules at its centre not radiated. Diameter 

 1 -432nd. Alive at Niagara. 



Genus Stephanogonia. — This is one of the new genera formed by 

 Ehrenberg out of his original genus Pgxidicula. Along with the 

 characters of Mastogonia, its valves have tnxncate apices, and spinous 

 angular umbilici. 



