12 ALGAE IN PHYTOPLANKTON OF THE UNITED STATES 



32B Cells and lorica without treelike ap- 

 pearance 33 



33A Colony discoid, one cell in thickness; cells in 



concentric rings Pediastrum 



33B Colony not discoid 34 



34A Colonies spherical or globose 40 



34B Colonies not spherical 35 



35A Colony with elongate cells radiating from 



common center Actinastrum 



35B Colony with cells not radiating from common 



center 36 



36A Colony with (generally) four to 

 eight cells positioned in linear 



series Scenedesmus 



36B Colony with cells not in linear series 37 



37A Colony with arcuate to lunate cells with 

 apices acutely pointed; convex faces of 



cells apposed Selenastrum 



37B Colony with spherical to broadly ellipsoidal 



cells 38 



38A Cells without spines or setae Crucigenia 



38B Cells with spines or setae 39 



39A Cells quadrate, closely apposed; free face of 



each cell with one or more spines Tetrastrum 



39B Cells quadrate and united; free face of each 

 cell with two to seven long delicate 



setae Micractinium 



40A Colony with biflagellated cells Pandorina 



40B Colony with nonflagellated cells 41 



41 A Cells lunate to sickle shaped Kirchneriella 



41 B Cells spherical or nearly so 42 



42A Cells borne terminally on 



dichotomously branched threads Dictyosphaerium 



42B Cells not on dichotomously 



branched threads 43 



43 A Colony a hollow sphere Coelastrum 



43B Colony not a hollow sphere 44 



44A Colony surrounded by partially 

 gelatinized and greatly expanded 

 parent cell wall Oocystis 



