1885.] "" [Stokes. 



tion, the whole to be finally withdrawn into a rounded, emarginate or 

 otherwise modified border. Consequently the Infusorian has the ability, 

 by the extrusion of a caudal prolongation, to come very close to a Uro- 

 leptus, and by the withdrawal of the tail to return to its proper generic 

 position. 



The lamelliform marginal setae, as they approach the posterior extremity, 

 gradually leave the ventro-lateral border and are developed on the dorsal 

 surface in a single row passing about the posterior part at a sbort distance 

 from the margin. The utility of this arrangement it is difficult to imagine, 

 unless it is to accommodate the anal aperture. Near the center of the 

 dorsal aspect, in addition to the numerous, immotile hispid hairs arranged 

 in longitudinal lines, there are developed three long flattened setae, volun- 

 tarily vibratile and resembling those on the body-margin. Such an addi- 

 tion to the dorsum of an Hypotrichous Infusorian has not been previously 

 observed. If somewhat more luxuriantly developed, these dorsal appen- 

 dages might, indeed in their present condition they do, lead to interesting 

 suggestions in respect to the affinities of the Infusorian and its order with 

 the Heterotricha. 



The large adoral cilia somewhat abruptly change their position in rela- 

 tion to the peristome-field as they approach the center of the left-hand 

 border, the free extremities of those most anterior being directed toward 

 the body-margin, while the tips of the posterior ones are vibrated above 

 the peristome-field, the alteration at the point of transition being quite 

 sudden. I have long suspected that this might be the arrangement in 

 other peristomal Infusoria, but have not been previously able to demon- 

 strate it. 



In its movements the creature is erratic. Remaining for a time quietly 

 lying with the ventral aspect upwards, suddenly with a lunge like that ot 

 a microscopic cetacean it rolls over, and exposes the dorsal surface only to 

 almost immediately begin a series of wild and grotesque backward tum- 

 blings, varying these acrobatic performances by rapid backward swim- 

 ming, occasionally throwing a backward somersault. It was the broad 

 marginal setae and these curious movements that suggested the name as 

 the flat-haired animalcule that tumbles over backward. 



Platytrichotus {^Xaruq^ broad ; rp^ioru^^ haired), gen. nov. Animal- 

 cules free-swimming, soft and flexible, more or less flask-shaped, widest 

 and inflated posteriorly, narrowest and depressed anteriorly, the ventral 

 surface flattened ; frontal styles five, uncinate ; ventral setae in two straight 

 median lines ; anal styles none ; marginal setae broad, flat, uninterrupted ; 

 nucleus single ; contractile vesicle single, near the center of the left-hand 

 border. Inhabiting fresh water. 



Platytrichotus opisthobolus, sp. nov. (Fig. 12). Body flask-shaped, less 

 than three times as long as broad ; frontal margin rounded, lip narrow, 

 crescentic ; posterior extremity soft and changeable in shape, obtusely 

 pointed, emarginate, bifid, but usually evenly rounded ; frontal styles five ; 

 ventral setae in two median rows, increasing in length posteriorly, those 



