GENUS GASTROSTYLA. 783 



ventral setae ; the two right-hand anal styles removed towards the posterior 

 extremity, and projecting to a considerable distance beyond its margin ; 

 contractile vesicle developed close to the posterior angle of the peristome-field. 

 Length of body 1-120" to 1-60". HAB. Fresh water, among Callitricha. 



Pleurotricha lanceolata, Ehr. sp. PL. XLIII. FIG. 27. 



Body elongate-lanceolate, two and a half times as long as broad, pointed 

 posteriorly, the anterior end curved slightly towards the left ; the supple- 

 mentary ventral setae less developed than in the preceding species, forming 

 a single complete and the half of a second row only on the right-hand 

 side of the larger ventral uncini, no such setae developed on the opposite or 

 left-hand side of the ventral area ; anal and ventral styles, peristome-field, 

 and other details, as in P.grandis. Length 1-144" to 1-84". 



HAB. Fresh water, among aquatic plants. 



This species is identified by Stein with the Stylonychia lanceolata of Ehrenberg. 

 The accompanying figure is illustrative of the encysted condition of the animalcule 

 as delineated by the first-named authority. 



Pleurotricha echinata, C. & L. sp. PL. XLIII. FIG. 22. 



Body elongate-lanceolate, rather more than four times as long as broad, 

 the extremities rounded, the right side nearly straight, the left one gibbous ; 

 peristome-field produced backwards to a little more than one-quarter of 

 the length of the whole body ; one row of ventral setae developed on each 

 side of the five scattered ventral uncini ; marginal setae forming a continuous 

 peripheral fringe, long, slender, situated far apart from one another, and 

 projecting at a right angle from the cuticular border ; anal styles five in 

 number, forming two groups, the two right-hand styles projecting posteriorly 

 and directed to the right ; contractile vesicle situated near the centre of 

 the left-hand lateral border. Length 1-275". 

 , HAB. Fresh-water ponds, in the Berlin Zoological Gardens. 



This animalcule, which is described by Claparede and Lachmann under the title 

 of Stylonychia echinata, is referred with some doubt to the present genus. It is not 

 improbable that the peripheral fringe of fine hair-like setae are of a supplementary 

 character, and homologous with the minute hispid setae which occur in a similar 

 position in Stylonychia mytilus and other Oxytrichidae, and in which case the two 

 rows of so-called ventral setae represent the true marginal series in this instance set 

 far inwards as in Amphisia. Should this premise prove correct, this form will 

 furnish the type of a new generic group to which, with reference to the characteristic 

 marginal fringe of setae, the title of Mesotricha might be appropriately applied. 



GENUS XVIII. GASTROSTYLA, Engelmann. 



Animalcules free-swimming, persistent in shape, elongate or elliptical ; 

 frontal styles usually five or six in number, the three anterior being most con- 

 spicuously developed ; ventral setae forming a single oblique row extending 

 from the right-hand border towards the anal styles, occasionally supplemented 

 by a few isolated central uncini ; anal styles five or six in number ; 



