288 DESCEiPTiON OF [Poli/ffastrica. 



Var. (b.J cmciatum. — Cells angular at the base, connected in a 

 crucifonn manner without a central space. 



Pediastkttm pertusum. = Micrasterias Boryana and M. trmjclia 

 (Elu'.) — Inner cells leaving hyaline intervals resembling foramina; 

 outer cells rectangular ; notch triangular, as broad as the cell. Frond of 

 from one to three circles arranged round one or two central ones. 

 The number of cells, as in P. Selenma and P. JBoryanum, generally 

 consists of five in the first circle, ten in. the second, and fifteen in 

 the third ; but Mr. Ralfs has seen fronds with four cells, forming a 

 circle aroxmd two central ones. 



P. granulatum. — Cells granulated ; lobes of marginal cells tapering 

 Frond of six cells arranged around two central sub-quadrate ones. 



P Jsapoleonis. ^^Micrasterias Napoleonis (Ehr.) — Six angoilar 

 cell-;, foiming a circle around two central ones ; lobes of marginal 

 cells arspidate ; notch wide. If not a variety of P. Boryanum ? 

 (P. 2. f. 117, 118.) 



p. angulosum, = Micrasterias angnhsa (Ehi\) — Marginal cells with 

 angular lobes, which are not extended into rays ; interstices hyaline. 

 Frond minute, of one or more cu'cles around a single central one. 

 The inner cells are angular, as in P. Boryana and P. eUipticum, but the 

 marginal are less deeply notched, and the lobes nearly angular, not 

 in the least extended into processes or rays. 



P. Boryanum. = Micrasterias Boryana (Elii'.) — Cells in one or more 

 circles aroimd one or two central ones ; marginal cells gradually 

 tapering into two long subulate points; notch narrow. (P. 2. 

 f. 115, 116.) 



It differs from P. ellipticmn, in the gradually tapering acute lobes of 

 the outer ccUs; from P. angidosKm in. itsloheshemg elongated as rays. 

 P. cUi^iticmn = Micrasterias elliptica (Ehr.) — Cells variable in 

 number and arrangement ; lobes of marginal ones suddenly con- 

 tracted into short, cyhndrical, obtuse processes. Frond large. 

 Yar. {h.) — Processes of the lobes truncato-emarginate. 

 P. senaria (Ehr.) — Cells in two concentric circles around one in 

 the centi'e ; twelve cells in the outer and six in the inner circle. 

 External cells without appendages. 



The two folio-wing new genera by M. Corda, are here introduced, as 

 they appear allied to Mr. Palfs genus Pediastrum. 



Cenus AsTEiiODiCTYON (Corda.) — Compound ; made up of many, but 



