442 ORDER CILIO-FLAGELLATA. 



maintained. In common with these simpler Flagellata, it is further noteworthy that 

 they often occur in such prodigious numbers as to impart a distinct tint to the water 

 they inhabit. Interesting data connected with this last-named circumstance will be 

 found recorded in the descriptions hereafter given of Peridinium sangiiineum and 

 Melodiniuni tiberrivitun. 



Genus I. HEMIDINIUM, Stein. 



Animalcules free-swimming, persistent in form, but not encuirassed ; 

 flagellum single, supplemented by an adoral fringe of cilia which is 

 developed in a groove that extends half-way only round the circumference 

 of the body, and at one extremity of which groove the oral aperture is 



located. 



Hemidinium nasutum, Stein. 



Body much compressed, somewhat kidney-shaped, the left border 

 straight, the right one convex ; the equatorial, semicircular, ciliated furrow 

 continued obliquely backwards from the ventral towards the dorsal region of 

 the left body-half; the anterior moiety of the ventral surface further dis- 

 tinguished by a faintly impressed vertical groove, the bottom of which gives 

 origin to the single flagellum ; endoplast roundish, located in the hinder 

 body-half. Dimensions unrecorded. 



Hab. — Fresh water ; colour yellow. 



This as yet single known representative of the genus Hemidinium is briefly 

 described without accompanying figures in the text of Stein's recently published 

 volume.* The oral aperture, though not directly observed, was adjudged to be located 

 close to the ventral extremity of the semicircular ciliated groove, while the capacity 

 of assimilating solid nutriment was fully proved by the presence of incepted green 

 corpuscles of considerable size. The integument in this type is reported by Stein to 

 be of firm consistence, and although not indurated and differentiated to such an 

 extent as to partake of the nature of a cuirass or carapace, exhibits a marked 

 contrast in this respect to the comparatively soft and yielding cuticular surface of 

 Gymnodinium. 



Genus II. GYMNODINIUM, Stein. 



Animalcules free-swimming, more or less persistent in form, but not 

 encuirassed ; cilia forming a continuous fringe along the interior of a 

 transverse groove or furrow which completely encircles the equatorial 

 region of the body, a second non-ciliated groove produced mesially on the 

 ventral surface from the transverse furrow towards the anterior or apical 

 extremity ; oral aperture and the insertion of the single, long, lash-like 

 flao-ellum located ventrally, in close vicinity to the juncture of the transverse 

 and longitudinal grooves. Inhabiting salt and fresh water. 



This genus is instituted by Stein t for the reception of several forms, including 

 notably the as yet very imperfectly observed Peridinium fuscum and pulvisculus of 

 Ehrenberg and the Peridinium monadicum and P. corpusculum of Perty, in which the 

 body and locomotive appendages, while according with those of the ordinary /lr/V//>//a, 

 differ from these in the entire absence of an indurated cuticular investment or cuirass. 

 As a consequence of this naked and comparatively soft consistence of the external 



* ' Infusionsthiere,' Abth. iii., 1878. t Ibid. 



