92 
BULLETIN OF THE UNITED STATES FISH COMMISSION. 
72. D. ludwigii Eckstein ('83). (Plate 20, fig. 32.) 
On aquatic plants in Put-in Bay Harbor, Lake Erie. 
The specimens found agree precisely with D. oxycauda as described by Stenroos (’98). This author 
holds that the differences between the specimens found by him and Eckstein’s figure of D. ludwigii 
are sufficient to justify describing them as a new species. These differences concern chiefly the form 
and distribution of the facets on the dorsal surface, and the shape of the toes. But Eckstein’s figure 
certainly gives the impression of trying to represent the facets only in a most general way, without 
attention to detail; and as for the toes, a study of Eckstein’s other figures, of known forms, shows 
that he made little attempt to be precise in his representation of such external characters. I therefore 
agree with Weber (’98) in considering D. oxycauda a synonym of D. ludwigii. 
Lake St. Clair (Jennings, ’94). 
73. D. stokesii Pell. (Plate 20, fig. 31.) 
On aquatic plants in Put in Bay Harbor, Lake Erie. 
Pell (’90) describes this form without giving the locality where found; it was probably at High- 
land Falls, N. Y. Lake St. Clair and Chippewa Lake, Mecosta County, Mich. (Jennings, ’94). Waters 
connected with the Illinois River at Havana, 111. (Hempel, ’98). 
74. D. flexilis Gosse. 
In Cliaracece from Put-in Bay Harbor, Lake Erie, and from East Swamp, South Bass Island. 
D. signifera .Jennings. — West Twin Lake near Charlevoix, Mich. (Jennings, ’96). 
D. inermis Bryce. — Sphagnum swamp near Pine Lake, Charlevoix, Mich. (Jennings, ’96). 
D. spinigera Western. — Sandusky Bay, Lake Erie (Kellicott, ’97). 
D. hornemanni Ehr. — Waters connected with the Illinois River at Havana, 111. (Hempel, ’98). 
D. minnesotensis Herrick. — This is an unrecognizable species; it is said by Herrick (’85) to occur 
“in America,” — from the name, doubtless in Minnesota, 
MONOSTYLA Ehr. 
The species of this genus have fallen into confusion that seems almost inextricable; the genus is 
in great need of a critical revision from a single standpoint. Four species of Monostyla are very 
common almost everywhere, and the same four species are to be found frequently described and figured 
in the literature of the subject. Four specific names are usually distributed among these species — 
M. quadridentata Ehr., M. lunaris Ehr., M. cornuta Ehr., and Ilf. bulla Gosse — but the names and figures 
are joined together in the most varied ways. I give herewith figures of these four species (figs. 35 
to 41) and will attempt by analysis of previous accounts to show the proper name to be applied to each. 
The name Monostyla quadridentata Ehr. unquestionably belongs to the form shown in fig. 40, 
plate 21. This species is so strongly marked by the two great spines at the anterior margin of the 
lorica that confusion with any other species is almost impossible. 
As to M. lunaris there is much confusion in the literature. Hudson and Gosse (’89) figure under 
this name the rotifer, a ventral view of which is shown in my fig. 41, plate 21. It is possible also that 
the M. lunaris figured by Levander (’94) is the same, though he represents the animal as having two 
claws at the end of the toe. 
Eckstein (’83) and Weber (’98) figure under this name an entirely different animal — that shown in 
my figs. 37, 38, and 39, plate 21. 
Referring to the original description of M. lunaris by Ehrenberg (’38), we find that the chief dis- 
tinctive feature of this species is the lunate concavity at the front of the lorica — “ fronte lunatim 
excisa.” This character gives the specific name lunaris, and Ehrenberg’s figures show a broad cres- 
centic inward curve from one lateral angle to the other at the wide front edge of the lorica when the 
animal is retracted. This shows that the animal called M. lunaris by Eckstein (’83) and Weber (’98) 
can not possibly be that species, as it lacks precisely the distinctive feature that gives the name to 
the species — namely, the crescent-shaped concavity at the front edge of the lorica. No matter how 
much contracted, this animal never shows a crescentic curve at the anterior margin; the actual con- 
tours at the anterior end in retraction are shown in my fig. 39. On the other hand, the animal called 
M. lunaris by Gosse (and Levander?) has this crescentic curve as the anterior margin of the lorica; 
hence I accept their determination as correct. This same animal (fig. 41) seems to be that figured by 
Eckstein (’83) as M. cornuta. 
In regard to M. cornuta Ehr., its distinctive features, according to Ehrenberg (’38), are the oval 
form of the lorica, nut deeply excised in front. Ehrenberg mentions also its remarkable resemblance 
to Cathypna tuna, almost the only difference between the two animals being the presence of two toes 
