Naviculacea-I infusoeial animalcules. 509 



but no central cell. Diameter l-1080th. In African Guano, con- 

 sidered as fossil. 



There is, too, a variety with five spines, of which two are smaller ; 

 andrliave the inner margin of the cells dentate. Diameter 1 -648th. 

 This may be another species. 



DicTTOCHA diommata. — Hexangular, with spinous angles ; eight 

 internal cells, and two centi'al, unarmed. Diameter 1 -660th. Fossil 

 in Virginia. 



D. Staur acanthus. — Eight-sided; spinous angles alternately longer ; 

 five inner cells, one median and four marginal, each armed with a 

 tooth. Diameter 1 -648th. Fossil. Virginia and Connecticut. 



D. triommata. — Hexangular, with spinous angles ; nine inner un- 

 armed cells; three median. Diameter 1 -864th. Virginia. This 

 species, and also D. diommata, resemble, in their turgid habit, D. 

 hemispherica, of which they may be but varieties. ■] 



D. gracilis (Kiitz.) — Slender, small, with elongated slender spines, 

 cells internally unarmed. Resembles in the main D. Speculum. 



Genus Mesocena. — Lorica simple, siliceous, univalve, occurring as 

 a ring, perfectly circular, or angular, and often armed with pro- 

 jecting spines (teeth.) 



This genus resembles JDictyocha, but is destitute of its central 

 organization. Ehrenberg remarks that Mesocena would be referrable 

 to the famUy Desmidiacea, had not that family a membranous lorica, 

 whilst it possesses a siliceous one. 



This genus, in Kiitzing's arrangement, belongs to the family 

 Actiniscece. 



M. Circuliis. — A single circular cell with a dentated margin- 

 Diameter 1 -576th. Fossil in Greek marl. 



M. elliptica. — A single elliptical cell obscurely quadrangular, with 

 four spines disposed in a quadi-ate manner on its margin. Diameter 

 l-624th to l-456th. Fossil in marl from the island of Zante. 



M. triangula — Triangular, with rough sides, and mucronate apices. 

 Fossil in chalk marl. 



M. Liodon. — In the form of a smooth elliptic ring, armed at each 

 end with a small tooth. Diameter l-396th. Maryland. 



M. SpongioUthis. — An elliptic ring with four slight alternating 

 swellings. Diameter l-492ud. 



