Desmidiece.'] infusorial animalcules. 285 



gelatinous covcriug, "whicli thus retains the two newly-partccl cells 

 together for some time longer, a fact wliich convincingly proves that 

 this genus belongs to the Desmidiece. 



SpiiioTiENiA eondcnsata. — Endochrome, a single, broad, closely 

 spiral band. Frond bright yellow-green, cylindrical. Length 1-20 8th. 

 (P. 13, f. 4.) 



S. ohscura. — Endochrome at first in several slender spiral threads, 

 afterwards uniform. Frond dark green; extremities attenuated. 

 Length 1 -247th. to 1 -226th. 



Genus Ankisteodesmus {Corda.) — CeUs elongated, attenuated, en- 

 tire, fasciculated. Cells fusiform or crescent-shaped, with no con- 

 etriction. They resemble Closterium, except in their aggregation. 



A. falcatus = X/mthidium (?) dijforme (Ehr.) — Cells very minute, 

 slender, crescent- shaped, fasciculated in irregular bundles, the indi- 

 viduals of which, having their convexity turned inwards, diverge at 

 each exti-emity of the bundle. Length of cell 1 -549th. (P. 2. 

 f. 3 ; and P. 12. f. 513. 514.); the two latter fossil specimens. 



A. fusiformis (Corda.) 



A. convolutus (Corda.) 



Genus Pediasteum (Meyen.) — Frond plane, circular, composed of 

 several cells, which form by their union a flattened star, and are 

 generally arranged either in a single cu'cle, or in two or more con- 

 centric ones ; marginal cells bipartite. The cells are combined into 

 a frond by a mucous matrix, which is generally colouiiess and con- 

 stitutes hyaline interstices. 



" The flat star-like fronds of Pediastrum are sufficiently cha- 

 racteristic. It is far more difficult to distinguish its species. 

 Ehi-enberg relies chiefly on the number of the circles, but this 

 character cannot always be depended on ; for it has been observed 

 that the number of cells is in the same species liable to great 

 variation; so also are the number of circles, and the number of 

 cells in the inner circle. Though so uncertain, we must still, for the 

 most, depend on the above characters in discriminating between 

 nearly allied species. Ehi'enberg, having relied almost entirely on 

 the number of circles, and the cells in each, has neglected the form 

 of the cells, and, consequently, his species are intermixed- transition- 

 states of some being referred to others." 



