ROTATORIA OF THE UNITED STATES. 
303 
the Anurceadce and Brctchionidce. In the latter families individual variation is so 
great that it is often almost impossible to determine whether two considerably 
differing specimens should or should not be considered different species. In the 
Rattulida ?■ variation does not extend to such lengths as this, and with good specimens 
the species may usually be recognized with much certainty. Of the 29 species 
which are described in the following from my own observation, there are, I am 
convinced, only one or two cases where later investigation may possibly unite two 
into one. One of these is that of Rattulus rattus Muller and Rattulus carinatus 
Lamarck, which has always been considered doubtful. Two very distinct forms are 
certainly found — one with a ridge, the other without — and I have not succeeded in 
showing that the two are really identical. Further, the species grouped about 
Diurella tenuior Gosse are rather critical as to specific distinctions. 
Points to be noted in descriptions of the Rattulidce . — From many of the descrip- 
tions of the Rattulidce given in the literature, it is exceedingly difficult to determine 
the animal in question, but this is due to the fact that the characteristic distinguish- 
ing features of the animal have not been noted. It will lie well to point out, there- 
fore, the features that are of especial importance in distinguishing species, and that 
should fie included, if possible, in ever} 7 description. I give them in the order of 
their importance. 
1. The teeth at the anterior margin of the lorica, their absence or presence, 
their number and relative size, if present. Those at the dorsal or dorso-dextral 
margin should be clearly distinguished from others due to the folding of the head- 
sheath when retracted. 
2. The toes, their length relatively to the body and to each other; their position. 
3. The general form of the body. 
4. The absence or presence of the longitudinal. folds in the head-sheath; their 
form, especially when the lorica is retracted, and any other characteristics of the 
anterior margin of the lorica. 
5. The “striated area,” whether developed as a single or double ridge, a furrow, 
a smooth area, or not developed at all. 
As many other features should, of course, be added as possible, but the above 
are the most important ones and should not be omitted from descriptions of any of 
the Rattulidce. Of course, an accurate figure or figures (showing the above points, 
as well as others) is perhaps even more important than a good description. 
Several of the most important points above mentioned, notably the presence, 
number, and relative size of the teeth at the anterior margin of the lorica, and the 
relative length of the toes, have very usually been omitted from specific descriptions 
of the Rattulidce; this makes it very difficult to recognize the animals. 
The following systematic account of the Rattulidce is arranged thus: I first give 
the characteristics of the family. This is followed by a key to the genera and 
species, which may be of assistance in locating quickly a given species; though for 
a determination, of course, the entire description and the figures should be studied. 
Some of the poorly described or doubtful species, which I have not myself seen, 
could not be taken into the key owing to the uncertainty as to important technical 
characters; these, however, are referred to at appropriate places in the key. 
Then follows a description of all the well-founded species of Rattulidce under the 
two genera. Under each genus I divide the descriptions into two parts, the first 
