THE OPALINID CILIATE INFUSORIANS. 85 



the other three. The nuclei of the broad subspecies are smaller than 

 those of the species proper. 



At first thought it seems that this multinucleated species and sub- 

 species should be assigned to the genus Cepedea^ but more careful 

 comparison seems to show them to be multinucleated Protoopalinae. 

 The nuclei are too large for Cepedea^ and especially the small number 

 of massive chromosomes, four, seems to place this form, beyond much 

 doubt, among the Protoopalinas. Protoopalina formoscG^ P. quad- 

 riniideata^ and P. axonucleata seem to form a series showing the 

 acquirement by the most highly modified Protoopalinas of a multi- 

 nucleated condition. In P. axonucleata the nuclei are of an even 

 number except in cases in which one of the nuclei has delayed its 

 division. In most individuals the nuclei are still united in pairs by 

 connecting threads. In some other individuals the nuclei of each pair 

 have separated. In general the daughter nuclei from the last mito- 

 sis do not separate until they enter upon the subsequent mitosis, but 

 the separation may occur earlier. 



It will be seen later that Protoopalina axonucleata is to be regarded 

 as the most highly evolved of the Protoopalinas, if my interpretation 

 of the speciation is correct. 



Genus ZELLERIELLA. 



The Protoopalinas — that is, the species of Opalinidae thus far 

 described in this paper — are all more or less cylindrical. They are 

 circular or broadly oval in cross section and are somewhat elongated. 

 There is another genus, Zelleriella^ whose species, while binucleated, 

 are narrowly oval or flat in cross section. Their nuclei are for the 

 most part spherical or nearly so, and they do not show in the different 

 species quite so much diversity in mitotic condition as we observe 

 among the Protoopalinas described. It seems well to group all these 

 flattened forms together under a distinct genus. This seems the 

 more justified from the fact that we know few transitional forms 

 between the two genera. After preliminary study of my series of 

 Opalinids, I used the genus name Protoopalina to cover both the 

 cylindrical and flattened binucleated species, and I have published 

 this classification (Metcalf, 1918, a and h). After more detailed 

 study, it seems both more convenient and a clearer expression of the 

 true conditions to separate the flattened forms under a distinct genus. 

 I am naming this genus Zelleriella after Zeller, whose memoir upon 

 the reproduction and development the Opalinidae, published in 1877, 

 is one of the best of the earlier papers on this group. Diagnostic 

 description of this genus, like that of Protoopalina^ is postponed until 

 we have seen in detail the character of the several species in the 

 family. 



