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margin rounded, not produced nor pointed, and without a dorsal cleft ; 
viewed ventrally, the cleft is seen not to be continued throughout the 
entire length of the lorica, but to he in two distinct portions, an an- 
terior short and posteriorly acute region, rapidly widening anteriorly, 
its margins convex ; and of a posterior portion, the anterior region of 
which is narrow, the borders being parallel for about one-fourth of 
their length, whence they rapidly diverge to form an elongate-ovate 
opening for the passage of the foot (fig. 8) ; viewed laterally (fig. 7), 
the frontal border is convexly truncate ; the dorsum strongly convex 
posteriorly, and somewhat flattened anteriorly ; eyes two, placed 
below the median line of the body ; foot with four joints, especially 
distinct in the dead animal ; toes two, acute. Length of lorica alone, 
1 /450 in. ; transverse width about 1 / 1300 in. ; toes about one- 
half as long as the foot. 
Habitat, shallow clear-water pools near Trenton, New Jersey, 
U.S.A. Abundant in its locality. Movements exceedingly active and 
exasperating. 
Near the middle of the hack is conspicuously displayed the inex- 
plicable organ referred to by Gosse in his account of Colurus deflexus 
Ehr., in which he says that, “ in the middle of the hack, just under 
the lorica, are two curious organs, each apparently an agglomeration 
of minute, clear vesicles, perhaps of air, perhaps of oil, observed long 
ago by Ehrenberg. He declared them inexplicable, and I cannot 
supply the explanation.” In the present form there is hut one organ 
of the kind, hut it consists of an irregularly subcircular ring of small 
refractive vesicles, apparently of oil, bordering a semi-opaque disc of 
granular matter varying in depth of opacity and of colour, as well as 
in degree of development, in different individuals (fig. 7). 
The frontal hook is strongly curved posteriorly, or at that part 
which is inserted, actually hooked, into the substance of the head, as 
shown by diagram in fig. 9. It is not indented in front. 
This Rotifer’s movements are so incessant and so annoying to 
the observer that, from the microscopist’s point of view, a stronger 
word might he a more appropriate specific name than the one 
selected, but the creature is assuredly agile. 
There is a variety of Colurus , presumably a variety of this species, 
found in company with it and often on the slide at the same time. It 
is rather larger and more robust than the typical form (1/375 in. 
in length without the frontal hook and the foot) ; the dorsal curve of 
the lorica is more regular ; the ventral borders are slightly more con- 
vex ; the posterior border rather more conspicuously emarginate ; the 
frontal hook is a broad, curved lamella indented at the free extremity, 
somewhat as in Colurus bicuspidatus Ehr. ; and the two eyes are 
usually placed above the median line of the body, or, at least, not 
below it, as in the typical Colurus agilis . 
