438 Dr. A. C. Stokes on some 



developed in a more or less concentrated infusion of fallen 

 leaves with water from the Delaware river. They are indi- 

 viduals from an interesting class of animal life particularly 

 abundant in this country, but one whose study has here been 

 almost entirely neglected. 



Heteromita mutabih's, sp. nov. (PI. XV. fig. 16.) 



Body ovate or subpyriform, about twice as long as broad, 

 very soft, flexible, and changeable in shape, the alterations 

 confined chiefly to the posterior extremity, and consisting of 

 varied pseudopodial extensions of this part, which is normally 

 widest and rounded ; endoplasm granular ; anterior vibratile 

 flagellum thick, slightly exceeding the body in length, the 

 posterior or trailing one slender, twice to two and a half times 

 that length ; nucleus obscure, apparently subspherical, near 

 the centre of the left-hand border ; contractile vesicle near 

 the right-hand margin of the posterior extremity. Length 

 of body -gfajs inch. 



Hab. Standing pond-water, with aquatic plants. Move- 

 ments evenly and steadily forward. 



Although this infusorian can assume various shapes appa- 

 rently at witlj it is remarkable for the presence and variety 

 of the posterior protrusions of the body-sarcode. These are 

 usually almost constantly formed during the creature's pro- 

 gression, one scarcely disappearing before its place is taken 

 by another of different shape and length. The tips of these 

 pseudopodial prolongations seem to be adhesive, since they 

 appear to cling to the surface of the glass slide and to require 

 a slight effort for their release. The production of these 

 characteristic prolongations, which, so far as I am aware, 

 have not hitherto been observed in any other species of the 

 genus except //. lens (Mull.), S. K., when in a dying con- 

 dition, together with the posterior location of the contractile 

 vesicle, which, with this exception, is placed so close to the 

 rear in II. lens only, are of diagnostic value, and will readily 

 lead to the recognition of the infusorian. From H. lens, for 

 which it is hardly possible to mistake it, H. mutabilis can be 

 distinguished by its normally ovate or subpyriform contour, 

 but chiefly, apart from the posterior changes of shape, by the 

 diverse length and thickness of the tiagella. In H. lens the 

 latter are both equal in size and about equal in length, being 

 twice as long as the body. 



Petalomonas cannata, sp. nov. (PI. XV. fig. 14.) 

 Body broadly ovate, somewhat longer than wide, both 



