772 ORDER HYPOTRICHA. 



proposed for that type tlie generic title of Gotwstomum — which name, however, 

 so closely resembles those of Gonostoma and Gonostomus, already employed to 

 designate certain genera of Fishes and Mollusca, that the substitution of a new one 

 appears desirable. 



Plagiotricha strenua, Eng. sp. Pl. XLIII. Fig. 34. 



Body flexible and contractile, elongate-elliptical, about four and a half 

 times as long as broad, evenly rounded at the two extremities, but slightly 

 narrower anteriorly; peristome-field very narrow, confined to the left-hand 

 lateral border, extending backwards to nearly the centre of the body ; the 

 adoral cirri very long ; ten uncinate frontal styles ; one and a half oblique 

 rows of even-sized ventral setae, which are developed backwards from right 

 to left, the complete row only approaching the left-hand border at a dis- 

 tance of one-third of the entire length of the body from the posterior 

 extremity ; four anal styles, and two long fine caudal setae ; a marginal row 

 of very minute hispid sets developed on the right and left peripheral 

 borders, these in company with the ordinary marginal series, projecting most 

 conspicuously towards the posterior extremity ; contractile vesicle located 

 near the posterior extremity of the peristome ; endoplasts oval, two in 

 number. Length I-160". Hab. — Fresh water. 



In describing this form as a species of Oxytrkha, Engelmann • remarks upon 

 its general resemblance to Oxytriclia {Plagiotricha) affinis, from which it, however, 

 differs in the number and disposition of the frontal, ventral, and anal styles, and in 

 the presence of terminal or caudal setje. Although not actually observed, Engel- 

 mann is inclined to believe that the fine hispid seta; that form a second and scarcely 

 perceptible peripheral fringe are developed also, as in Oxytriclia parallela, through- 

 out the dorsal region. The anal styles in this species appear to be of a softer and 

 more plastic consistence than is usually found among the Oxytrichidae, two of them, 

 which are set a little in advance of the others, being flexible and subservient 

 for ambulatory purposes, after the manner of ordinary ventral setEe, which are here 

 unrepresented towards the posterior extremity of the body. 



Plagiotricha (Gonostomum) aCBnis, Stein sp. Pl. XLIII. Fig. 25. 



Body elongate-lanceolate, three and a half to four times as long as wide, 

 narrowed and pointed at each extremity ; peristome-field arcuate, confined 

 chiefly to the left-hand lateral border, but its posterior extremity bent 

 abruptly inwards and terminating near the centre of the body ; ventral setse 

 five or six in number, disposed obliquely from right to left, but not ex- 

 tending further backwards than the posterior termination of the peristome- 

 field ; the marginal setae forming an unbroken projecting peripheral fringe, 

 somewhat larger posteriorly ; anal setae very short and inconspicuous, not 

 reaching to the posterior extremity of the body ; no caudal setae. Length 

 1-288" to 1-216". Hab. — Marsh water. 



This type, representing the Oxytriclia affinis of Stein and Gonostomum affinis of 

 Sterki, is, as previously stated, retained in the present generic group in recognition 

 of its general conformity, excepting for the absence of caudal set£e, with the pre- 



* ' Zeitschrift fiir WissenschaftUche Zoologie,' Bd. xi., 1S61. 



