DesmiJiea.] infusokial animalcules. 287 



" 2. Tlio relations of size are not always in ratio witli those of 

 number; for we find small size with a large number, and great size 

 with a small number of parts. 



" 3. Fonns exhibiting equal relations of number to size, otherwise 

 distinguish themselrea in secondary and less important points, as in 

 a slender or thicker form of cell, long or short horns, &c., &c. ; con- 

 ditions varjing in almost every individual. 



"4. Of the numerous forms seen by me, all were two-pointed, 

 none had one-pointed bodies. (No Pediastrum simplex.) 



"5. llegularity of form is the character of integrity from ii-regu- 

 larity; no distinct species can be formed." (P. 2. f. 114 to 117, 

 121 to 123, and P. 13, figs. 22, 28, and 29, illustrate this genus.) 

 Mr. Halts divides Pcdiadnmi into five sections. 



* Lobes of the outer cells emargmate or truncate. 



PjEDiASTKUir tetras = Micrasterias tetras, (Ehr.) — Cells four, se- 

 parated by colourless interstices which form a cross ; lobes truncato- 

 cmarginate. Frond extremely minute. Length of cell 1-294 1th. 

 (P. 13. f. 27.) 



P. heptactis ^= Micrasterias heptactis (Ehr.) — Cells seven; six dis- 

 posed in a circle around a central one ; cells bipartite, with emar- 

 ginate lobes. Length of cell l-2906th. (P. 2. f. 114.) 



P. biradiatum = Micrasterias Rotula, (Ehi*.) Outer circle ge- 

 nerally of eleven bipartite cells, with bifid or emarginate lobes. 



Var. (b.) — Lobes of outer cells ti'uncato-emarginate. 



The five-angular cells forming the inner circle are often quadri- 

 lateral, and the exterior of each has a linear notch. Length of cell 

 of outer circle 1 -2000th. 



* * Lobes of the segments entire. 

 t Connecting substance coloured. 



P. Selenosa = Micrasterias Boryana (Ehr.) — Cells crescent- shaped, 

 arranged in one or more circles, around one or two central ones ; 

 connecting medium coloui-ed. Frond minute. 



1 1 Interstices of the frond hyaline. 



P. simplex. = Micrasterias coronula (Ehi-.) — Cells four, with or 

 without one or two central ones; lobes of marginal cells ovate, 

 tapering to a point. 



Var. {a.) — Marginal cells tnmcatc at the base and forming a circle, 

 its centre vacant, or occupied by one or two cells. 



