ROTATORIA OF THE UNITED STATES. 
281 
effect of tlie longitudinal folds in the head-sheath above described or to the slight 
rounded projections of certain parts of the head-sheath mentioned in the last para- 
graph. Sometimes the folds of the head-sheatli project as sharp teeth. An example 
of this condition is found in Diurella rousseleti Voigt, where there are nine of these 
teeth. These, however, are of different character from the one or two teeth which I 
have described above. These latter are structures to a certain extent sui generis , 
and I shall, as a rule, restrict the use of the term teeth in this connection to them. 
Stenroos (1898) lias described a new species, Mastigocerca ( Rcittulus ) rosea , which 
is said to have two long teeth or spines, like those of Rattulus longiseta Schrank, at 
the ventral margin of the lorica. In other respects the animal resembles Rattulus 
longiseta Schrank. As this peculiar position of the teeth is unknown in any other 
of the Rattulidce, and is entirely out of harmony with the structure and behavior of 
the Rattulidce in other respects (as will appear later), it seems possible that there 
was an error of observation in this case. 
Striated area , Ridge.— One of the most peculiar characteristics of the Rattulidce 
is the presence on the lorica of a dorsal longitudinal area, striated transversely, 
which extends from the anterior edge some distance backward on the body. This 
area shows the most varied differentiations in different species — in some appearing 
as a single high ridge, in others as two ridges, in others as a depression, while in 
still other cases there is no change in the surface of the lorica at this region except 
the transverse striations. This peculiar area is so characteristic for the Rattulidce, 
and plays such a part in determining their forms, that it must be treated in full. 
The area is unsymmetrical in position, usually beginning at the anterior margin 
of the lorica, to the right of the mid-dorsal line, and passing obliquely backward 
and toward the left side. Its sides are, as a rule, rather sharply defined, frequently 
appearing as thickenings or ridges. This area shows in Rattulus elongatus Gosse a 
condition which will serve as a useful point of departure for an understanding of the 
various differentiations which it undergoes in other species. In R. elongatus Gosse 
(pi. xii, fig. 102) the area begins at the anterior edge as a broad, shallow furrow, with 
well-marked sides. This furrow lies a little to the right of the position of the eye, as 
seen from above. From the sides of the furrow transverse striations pass toward 
its middle (and a little forward). The striations are not continuous from one side 
to the other, but meet in the middle of the furrow in a sort rtf rhaphe. 
The furrow proper extends backward for a distance only somewhat greater than 
the diameter of the lorica. Near its posterior end, in its middle line, is situated the 
dorsal antenna. Though the furrow or depression below the general surface ceases 
at the point above indicated (shown at x, figs. 102 and 105), the striated area con- 
tinues, with well-defined edges, for about one-third the length of the lorica. 
In Rattulus longiseta Schrank (pi. viii, fig. 67) the striated area is of very nearly 
the same character as in Rattulus elongatus Gosse, save that it exists as a depres- 
sion throughout its entire length, reaching to the middle of the lorica. In this 
species we have another characteristic feature added — the relation of the striated 
area to the two anterior teeth or spines. The two teeth are continuations of the 
thickened edges of the striated furrow. This appears to be true in all species where 
the teeth exist. The tooth or spine which forms the continuation of the right edge 
is much longer than the left one. 
What is the function of this striated area and what are the transverse striations 
which mark it? The striated furrow, as we find it in Rattulus longiseta Schrank, 
