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BULLETIN OF THE UNITED STATES EISH COMMISSION. 
2. Diurella tenuior Gosse (pi. i, figs. 7-10). 
Synonyms: Ccelopus tenuior Gosse (1889); Mast igocerca flectocaudatus Hilgendorf (1898). 
Distinguishing characters. — This species is to he known by its elongated curved body, with a 
single tooth at the anterior margin of the lorica, and the unequal toes (pi. 1, fig. 9), the right toe 
being only about one-half as long as the left or a little less than one-half as long. 
It has much resemblance to D. tigris Muller, from which it differs in the unequal toes and 
in certain other characters mentioned in the account of that species. It also greatly resembles 
Rattulus gracilis Tessin, from which it differs in the following particulars; The body in D. ten- 
uior is regularly curved, so that no straight outlines appear, as in Rattulus gracilis; the second 
toe is longer in proportion to the main one than in Rattulus gracilis, where the lesser toe is only 
about one-third the length of the main toe, while here it is about one-half the length of the latter; 
the head is much less sharply set off from the remainder of the lorica than in Rattulus gracilis; 
the tooth is more pronounced and the ridge less prominent than in the last-named species; the 
foot is shorter and less prominent than in Rattulus gracilis. 
Diurella tenuior Gosse also has a striking resemblance to Diurella insignis Herrick in the 
form and in the toes, but the latter species has two teeth at the anterior edge of the lorica in 
place of one, is much larger, and there are other differences in details. 
This species is related, finally, to D. intermedia Stenroos, but D. tenuior is usually much larger 
than D. intermedia and is longer in proportion to its diameter. The two differ especially, however, 
in the toes, those of D. intermedia being equal. 
External features. — The body is long and cylindrical, much as in D. tigris Muller. There is 
a low oblique ridge on the right-hand side, passing backward from the point of origin at the ante- 
rior tooth to about the middle of the length of the lorica. This is striated transversely, as in other 
species. The head-sheath is marked off from the remainder of the lorica by a slight constriction. 
It has longitudinal folds somewhat similar to those of D. tigris Muller, though perhaps hardly 
so prominent; by these the anterior opening of the lorica can be neai-ly closed when the head is 
withdrawn. On the dorso-dextral part of the anterior margin there is a tooth, perhaps hardly so 
prominent as that of D. tigris Muller, but rather more pronounced than in Rattulus gracilis Tessin. 
The corona has not been especially studied. 
Antennae. — The dorsal antenna (figs. 7, 8) lies in the usual position, a little to the left of the 
ridge. The right lateral antenna has the usual position on the posterior third of the body (fig. 8). 
The left lateral antenna I have not seen. 
Foot. — Rather broadly conical, not so sharply set off from the lorica as in D. tigris Muller. 
Toes (fig.' 9). — The two toes are unequal in size, the right toe being about half, or a little less 
than half, the length of the left toe. The main (left) toe is a curved, pointed rod, about half the 
length of the lorica. The right toe is much more slender and is so curved that its tip usually lies 
against the main toe at about the middle of the length of the latter. The right toe seems to be, as 
a rule, a trifle less than half the length of the main toe. This species, in its technical characters, is 
on the boundary line between Rattulus and Diurella, and is as closely related to Rattulus gracilis 
Tessin as to any of the species of Diurella. 
Just outside the base of the main toe is a substyle which is nearly as long as the right toe. 
There is also a minute substyle just outside the base of the right toe. 
Internal organs. — These call for no special remark, except in the case of the trophi (fig. 10). 
The trophi are very similar to those of Diurella tigris Muller and Rattulus gracilis Tessin — the 
right side being very rudimentary as compared with the left. 
Measurements. — Different specimens of this species vary excessively in size. Two specimens 
drawn to the same scale are shown in figs. 7 and 8 (pi. 1). The length of body varies from 0.135 
to 0.21 mm.; length of toes, from 0.055 to 0.08 mm.; total, from 0.19 to 0.29 mm. 
History. — This species was first described by Gosse in Hudson & Gosse ’s Monograph of the 
Rotifera (1889). Like many of Gosse T s descriptions, that of this species is inaccurate in some 
details. For example, he states that the head is defended by two or three projecting, points. W eber 
(1898) has likewise given a description and figure of this species, repeating Gosse’s statement that 
there are three or four points at the anterior edge of the lorica, though his figure shows but 
one. It is probably the longitudinal folds in the head-sheath that have given rise to the impression 
