450 
Transactions of the Society. 
X . — List of New Rotifers since 1889. 
By Charles F. Rousselet, F.R.M.S, 
( Read June 21 st , 1893.) 
The Supplement to Messrs. Hudson and Gosse’s great -work on the 
Rotifera was published early in 1889, and, together with the two 
previous volumes, contains an account of all species that were known 
to the authors to the end of 1888. The publication of this book has 
had an immense stimulating effect upon students of this well-defined 
and interesting group of animals, with the result that no less than 
186 new species have been described since that date. Almost all 
parts of the world have contributed to the number — from Greenland 
to Australia, and from China to America. Some of these so-called 
new species are, however, not new, and can be recognized as old 
friends, and some have been named twice over, while a number of 
others have been described and figured in so unsatisfactory a manner, 
that it will be quite impossible to recognize them again. It will be a 
difficult task to arrange all these species in the system and separate 
.the good species from those insufficiently known. A word of advice 
may not be out of place here to all workers in this field ; it is, Avoid 
as much as possible making new species. It must be borne in mind 
that a great many forms are imperfectly known, and have been 
described years ago, when the optical means at the disposal of 
observers were much inferior to those now available ; again, the 
animals have not always been observed under the most favourable 
conditions, and there can be no doubt that Rotifers, like most other 
animals, vary a little in different localities. It is therefore very 
natural that new details can sometimes now be seen which were over- 
looked or invisible before, and that other details are not exactly as 
originally described. Such corrections and variations should be 
recorded, but it is not necessary to make new species, especially in 
the more difficult and obscure genera, when animals are found which 
do not agree in every minute particular with the type. As a general 
rule a closely allied animal should only be described as new when the 
type itself is known to the observer. Descriptions of insufficiently 
observed animals should not be published ; it is better in such cases 
to make a note and await another opportunity. I am afraid Mr. Gosse 
has set a bad example in this respect, in naming a new species 
(Distemnia lahiatum) from a single observation, when the animal swam 
across the field of view and was then lost. 
But when a really new species has been found it should be figured 
and described in such a manner that the animal may readily be recog- 
nized when found again by a different observer, and a good figure is 
often worth more than a good description. A figure of the jaws 
should always be added. The true character of the jaws cannot 
