THE OPALIXID CILIATE IXFUSORIAXS. 175 



The ranaru?n-Yike forms Stokes describes, of course, can not be 

 identified. The species I have found in the same host is Opalbia 

 oh trigonoidea. 



Genus OPALINA. 



The uniformly and much flattened multinucleated forms have 

 now been reached. Among them we can distinguish two general 

 sorts, the obtrigona-like species, Opalinae caigiistae, in general oblan- 

 ceolate in form, and the rariannn-Wke, species, Opalinae latae^ more 

 rounded. Between the marked extremes of these two sorts are numer- 

 ous intermediate forms, so that the two subgeneric groups are not 

 sharply demarcated from -each other. It seems possible that these 

 much flattened, uniformly flattened species, the Opalinas, may have 

 arisen independently several times from cylindrical forms. Indeed 

 we have two, perhaps three, series of intergrades that suggest this. A 

 number of the Cepedeas are quite flat anteriorly, and some species are 

 also somewhat flattened even posteriorly. A multiple origin for the 

 much flattened multinucleate forms seems the more possible, since 

 among the binucleated OiDalinids there are, on the one hand, the cylin- 

 drical or slightly flattened Protoopalinas and, on the other hand, 

 the much flattened Zelleriellas. This tendency toward flattening al- 

 most surely appeared at least twice in the family, once among the bi- 

 nucleated forms, and once among the multinucleated forms. The 

 Zelleriellas may likely be monophyletic ; at least we have no special 

 indication of multiple origin. The flattening of Protoopalinas to 

 produce Zelleriellas seems a phenomenon parallel to the flattening of 

 Cepedeas to produce Opalinas, and seemingly the latter process may 

 have taken place more than once in the phyletic history. If this be 

 true, the term Opalina used as a generic name for the much flattened 

 species is not strictly a proper taxonomic term indicating true genetic 

 relationship, but is rather a convenient designation for species of 

 similar shape but probably somewhat diverse origin. We have not, 

 however, the data for determining the genesis of the several species, 

 so must retain the genus Opalina in this somewhat unscientiflc use. 



Among the ohtrigonaAikQ Opalince there is such intergradation 

 between species that here again, as among the Zelleriellas and Cepe- 

 deas, we can not always sharply and with certainty demarcate species, 

 and the ranaruni-Mko. species present similar difficulties. Any classi- 

 fication must be somewhat arbitrary and conventional. 



opalin.\p: a^'gustae. 



OPALINA OBTRIGONA Stein (1864). 



Specimens of this species have been deposited with the United 

 States Xational Museum as follows: Xos. 16533 and 1GG14 from 



