286 IU'dio'ih State Laboratori/ of Natural History. 



We also find that the structural dift'erentiation shown in the 

 decadence of its posterior stigmata ol>tains in varying degrees 

 in the other spherical and ellipsoidal genera of the family — 

 least in Pandorlna, most in Volvox. The genus I'lcoilor'nia 

 agrees with VoJrox in having a structural polarity hased upon 

 the division of the colony into vegetative and gonidial regions, 

 hut the differentiation is simpler. Of the two species of Pleo- 

 dor'uia, the one here described exhibits the simplest possible 

 differentiation of the colony consistent with the symmetry 

 of the organism; viz., the differentiation of the anterior 

 polar circle of four cells as vegetative members of the colony. 

 Of the two species of the genus it thus stands nearer Endo- 

 riiut, while its sister species P. californlca approaches more 

 closely to Volro.r both in the number of cells and in the 

 extent of the differentiation. 



The discovery of this additional species of the genus Plco- 

 dorlna thus supports the opinion expressed by Shaw ('94), 

 who founded the genus, that it W'as intermediate between 

 Eiiiloy'nia and ]'olvox but nearer the former. Judging merely 

 from the asexual stage, P. drniolsem^is affords additional evi- 

 dence of the close relationship of Plcodorina and Kudor'uia. 



Throughout the preparation of this paper the writer has 

 had constantly in mind the possibility that the form here 

 described is merely a stage in the life cycle of Kiidor'nta. A 

 number of facts lend support to this hypothesis : ( 1 ) the 

 occurrence of Pleodorlna Uiinoiseusis with Endor'nia elc(j((iiH ; 

 ('2) their marked similarity, aside from the four vegetative 

 cells, in structure and measurements; (3) the impossibility 

 of separating the youngest free-swimming colonies of the 

 two forms; (4j a considerable variation in the size of 

 the vegetative cells in Plcmior'nia, grading toward the 

 condition in Eudor'uia \ (5) some evidence that in certain 

 cases at least the vegetative cells may divide, one case of a 

 2-cell stage having been seen in the hundreds, if not thou- 

 sands, of sx^ecimens examined, and one instance noted in 

 which a maternal colony containing thirty-two daughter 

 colonies had at one pole four colonies whic,^ were slightly 

 smaller than the remaining twenty-eight; and (6) the occur- 



