NO. 2164. REVISION OF ROTATORIA^ GENERA— EARRING. 529 



of certainty be accepted as correctly determined, appear to be found 

 the world over, wherever collections are made in suitable localities, 

 and the relatively small number of existing records are consequently 

 of little significance. As for the more critical species, on account of 

 the lack of precision in the original descriptions, from which the 

 determinations were made, it is usually quite uncertain what the 

 investigator who recorded them may have been dealing with, and a 

 repetition of such references would be of no assistance in attempts to 

 ascertain the geographic range of the species. It is the hope of the 

 writer that this paper may simplify the task of identifying these 

 small forms and thus contribute to a solution of the problems of 

 distribution. Of the species here described there are at least 8, 

 and possibly 10, that have not as yet been reported from Europe; 

 whether they will eventually be found there also, or whether an 

 equal number of non-American forms will bo discovered, remains to 

 be seen. However, the latter contingency appears rather remote, as 

 the genus has all the earmarks of being a very ancient one, which 

 long ago became so highly specialized as to preclude any further 

 evolution or local species-formation and therefore devoted its 

 energies to dispersion. 



All the species described have been studied from fully contracted 

 specimens, as the anterior margin offers some of the most important 

 characteristics. In order to avoid needless multiplication of plates 

 and to indicate the limits of variation the dorsal and ventral views 

 have been drawn from different specimens. For some highly varia- 

 ble species this was insufficient and enough figures have been added 

 to show at least the principal forms likely to be met with. 



For the illustrations the shding scale used by the writer in two 

 earlier papers has been employed. An animal measuring 100 n is rep- 

 resented by a figure 50 mm. long and for each succeeding 100 ju an 

 addition of 10 mm. is made. This method is a compromise and as 

 such is of course open to objections, but it gives at least a system. 

 The uniform magnification used by some writers is objectionable in 

 that either the small species must be figured so very small as to make 

 it impossible to give the necessary detail, or else the large forms 

 reach impracticable dimensions. A uniform magnification seems, 

 moreover, comparatively meaningless; when natural size is once 

 discarded, the extent of the departure is of minor importance. In 

 the study of the animals a magnification suited to the object is em- 

 ployed and actual measurements obtained by means of the microm- 

 eter. The sliding scale has the advantage of not reducing the smallest 

 species below the point where details may be satisfactorily shown, 

 while the largest known rotifer may be figured on an octavo page 

 and at the same time some indication of the comparative size is given. 

 36399°— Proc.N.M.vol.51— 16 34 



