GENUS AMPHISIA. 767 



at the centre, the left angle of the anterior margin of the upper surface of 

 the carapace in adult individuals somewhat obliquely truncate, but in 

 younger ones symmetrically rounded and overlapping the ventral surface ; 

 peristome-field triangular, its inner or right-hand border reflected back 

 along the median line, bearing a band-like undulating membrane and a 

 row of fine cilia ; three rows of uncinate frontal styles containing collec- 

 tively from sixteen to twenty-eight elements, the three more anterior of 

 which are much the largest; from fifteen to twenty-one ventral styles 

 disposed in the three or four central rows, and from five to six or seven 

 large, straight, anal styles ; endoplasts ovate, usually four in number, each 

 having a minute attached endoplastule. Length 1-240" to 1-72". 

 HAB. Fresh water. 



GENUS VI. AMPHISIA, Sterki. 



Animalcules free-swimming, oval or elongate, not encuirassed, soft and 

 elastic ; the ventral surface more or less flattened, bearing three or four 

 frontal styles, two or three continuous median rows of ventral setae, and five 

 or more anal styles ; the marginal setae not projecting at the periphery but 

 forming two outer rows in close proximity to the ventral setae, with which 

 they correspond in size and character ; contractile vesicle single, spherical, 

 situated near the centre of the left-hand lateral border. Mostly inhabiting 

 salt water. 



The representatives of this genus have been recently separated by Sterki * from 

 the typical members of the genus Oxytricha as here amended, as also from those of 

 Holosticha, which they still more nearly approach, with reference to the modifica- 

 tion of their marginal setae. These structures are here removed to a considerable 

 distance from their normal peripheral position and stationed more or less towards 

 the centre of the ventral field ; their size and character at the same time correspond 

 so closely with those of the ventral series that they appear to be merely supple- 

 mentary rows of the same. A closely identical form and disposition of the marginal 

 setae occur in certain species of Uroleptus. 



Amphisia gibba, Mull. sp. 

 PL. XLIIL FIG. 15, AND PL. XLIV. FIGS. 15 AND 16. 



Body thick, elongate-elliptical, three and a half times as long as broad, 

 widest in the centre, smaller and equally rounded at the two extremities ; 

 the peristome-field narrow, fissure-like, exhibiting no reflected border, ex- 

 tending obliquely to the centre of the anterior third of the body ; ventral 

 setae forming two even, continuous, closely approximated central rows 

 the marginal setae similar in character, stationed so close to the ventral setae 

 as to present the aspect of a third or fourth median series; anal styles 

 five in number, sub-even, projecting for nearly half their length beyond the 

 posterior margin. Length 1-144". HAB. Salt water. 



Stein regards this species as corresponding with the Trichoda gibba, and 

 probably also the T, fceta of O. F. Miiller. An apparent variety of this type, 

 possessing an additional row of ventral setae, see PL XLIV. Figs. 15 and 16, 



* ' Zeitschrift liir Wissenschaftliche Zoologie,' Bd. xxxi., 1878. 



