GENUS HIS TRIO. 789 



of the ventral and marginal setae ; under these circumstances its retention among 

 the typical representatives of the genus Oxytricha must be regarded as provisional. 



Supplementary Species. 



Oxytricha tubicola, Gruber. PL XLIV. Fig. 12. This species is briefly described 

 by Dr. August Gruber,* in company with Stichotricha socialis and several other 

 interesting marine and fresh-water species. Excepting, however, for its habit of 

 excreting and inhabiting a short tube, open at both ends, as shown in the accom- 

 panying figure, the data recorded are insufficient to define its true position with 

 relation to the other members of the Oxytrichidae. The character of this protective 

 element, if confirmed by future observation, at the same time suffices to indicate the 

 necessity that will probably arise of instituting a new generic title for its reception. 



GENUS XXI. HISTRIO, Sterki. 



Animalcules free-swimming, persistent in shape, ovate or elliptical, 

 bearing three large uncinate and a few smaller setose frontal styles, five 

 scattered ventral and five anal styles; an uninterrupted projecting fringe 

 of marginal but no caudal setae ; peristome-field with a reflected inner 

 border ; endoplasts two in number ; contractile vesicle situated near the 

 centre of the left lateral margin. 



This genus is instituted by Sterki \ for the reception of the Stylonychia histrio of 

 Ehrenberg and more recent writers, which differs from the typical Stylonychia in 

 the entire absence of caudal setae. 



Histrio Steinii, Mull. sp. PL. XLV. FIGS. 13 AND 14. 



Body elongate-elliptical, somewhat lancet-shaped, slightly curved towards 

 the left ; the marginal setae continuous throughout ; no longer caudal setae 

 developed at the posterior extremity, and none of the anal styles project- 

 ing beyond the posterior border ; endoplasts ovate or nearly spherical. 

 Length 1-190" to 1-168". HAB. Fresh water, among aquatic plants. 



This animalcule is identical with the Kerona histrio of O. F. Muller, and the 

 Stylonychia histrio of Ehrenberg and Stein. The conversion of the old specific title 

 into a new generic one, and the adoption of a new name to replace the former as 

 accomplished by Sterki, is a course perhaps scarcely to be commended. 



Histrio similis, Quenn. sp. 



Body elongate-elliptical, straight, not quite three times as long as 

 broad, rounded at each extremity ; the marginal setae forming an even and 

 continuous lateral border ; no caudal setae ; anal styles five in number, 

 simple, straight, and slender, projecting for about half their length beyond 

 the posterior extremity. Length 1-225". HAB. Salt water. 



The straight and more regularly elliptical form of this species, in addition to the 

 prominence of the anal styles and salt-water habitat, distinguish this species from 

 Histrio Steinii, which it otherwise most nearly resembles. 



* ' Zeit. Wiss. Zool.,' Bd. xxxiii., 1879. Also translation, 'Journal Royal Microscopical Society,' 

 vol. iii., No. 2, April 1880. 



t 'Zeitschrift fur Wissenschaftliche Zoologie,' Bd. xxxi., 1878. 



