Key to the Genera 



1. Colony spherical or ovoid- ____ 2 



1. Colony a flat or twisted plate with cells in 1 layer 5 



2. Colony spherical, ovoid, or obovoid, containing 256 or fewer cells. 3 



2. Colony a hollow sphere, containing from 500 to several 



thousand cells . „ Volvox 



3. Colony containing from (32)— 64 to 256 cells, which are of 2 



sizes Pleodorina 



3. Colony containing less than 64 cells, all similar in size 4 



4. Colony ovoid or obovoid, composed of spherical or ellipsoid cells 

 arranged at some distance from one another and often 



appearing in transverse tiers . . ^. Eudorina 



4. Colony ovate, composed of pyriform cells compactly arranged Fandorina 



5. Colony a flat, rectangular plate of from 4 to 32 cells Gonium 



5. Colony horseshoe-shaped, slightly twisted on its 



axis, composed of 16 or 32 cells "Platijdorina 



GONIUM Mueller 1773; emend. Prescott in 

 Prescott, Silva, & Wade 1949, p. 84 

 A free-swimming, plate-like, quadrangular colony of 4-32 ovate, 

 ovoid, pyriform, or bilobed cells so arranged within a gelatinous 

 envelope that a rectangle of 4-8 central cells is inclosed by a periph- 

 eral series of 12-24 individuals; the long axis of the central cells at 

 right angles to the plane of the colony, but axis of peripheral cells 

 radial to the center of the colony. Cells interconnected by fine proto- 

 plasmic processes; individual envelopes adjoined by stout processes 

 with those of neighboring cells, so that oval or quadrangular inter- 

 stices are formed in the colonial mucilage. Flagella 2, attached in 

 the narrow anterior end just above 2 contractile vacuoles. Chloro- 

 plast a parietal cup with 1 or 2 pyrenoids; pigment-spot usually large 

 and conspicuous, lying laterally in the anterior end. Sexual repro- 

 duction by the division of the vegetative cells into 4-16- (32?) iso- 

 gametes which fuse in pairs, the quadriflagellate zygote becoming 

 a thick-walled resting spore. 



Key to the Species 



1. Colony 4-celled, with anterior ends of cells all directed outward .— G. sociale 



1. Colony 16-32-celled (rarely 4-celled); the outer cells with flagella directed 

 in the transverse plane of the colony, the inner cells with flagella directed 

 at right angles to the transverse plane 2 



2. Cells ellipsoid or subspherical G. pectorale 



2. Cells ovoid-pyriform, narrowed anteriorly G. formosum 



Gonium formosum. Pascher 1927, p. 418 



Colony of 16 ovoid-pyriform cells inclosed in relatively wide indi- 

 vidual sheaths which are joined to neighboring sheaths by narrow 

 and relatively long arm-like extensions so that each of the inner 4 



[74] 



