GENUS PERITROMUS. 759 



the remaining portion of the ventral surface dotted with fine vibratile cilia 

 only, never developing supplementary setae, styles, or uncini. 



The as yet single known representative of this family combines the 

 characters of the two groups of the Chlamydodontidas and Oxytrichida;, it uniting 

 the simply ciliate ventral surface of the former — as typified by the genus ChiloJon — 

 with the edentulate pharynx, excavate peristome-field, and arcuate fringe of adoral 

 cilia characteristic of the latter. Its passage to the ordinary Oxytrichidae is 

 clearly through the genus Kcrona, with which, except for the presence of vibratile 

 cilia in place of fine setse on the ventral surface, it would appear to closely corre- 

 spond. The isolation of Peritromus as the type of an independent family group, 

 upon which he confers the title of the Peritromina, is accomphshed by Stein in the 

 second volume of his ' Organismus.' 



Genus I. PERITROMUS, Stein. 



Animalcules free-swimming, rcniform, depressed ; peristome-field ex- 

 tended backwards to the centre of the ventral surface ; no ventral or 

 anal styles or setae; the whole under surface dotted with short fine cilia 

 which are distributed in closely approximated longitudinal lines ; endoplast 

 double ; contractile vesicle situated near the posterior edge of the peri- 

 stome ; anal aperture sub-terminal. 



Peritromus Emmse, Stein. 



Body reniform, widest posteriorly, about twice as long as broad, both 

 extremities equally rounded ; the right lateral margin convex, the left one 

 indented centrally ; peristome-field bow-shaped or arcuate, extending back- 

 wards to the median line, its outer edge bordered with long powerful 

 adoral cirri ; fine, short, vibratile cilia distributed in many parallel lines 

 over the remaining ventral surface. Dimensions unrecorded. 



Hab. — Salt water. 



In general contour, and in the form and character of the peristome, this species 

 closely resembles Keroua polyponmi ; added to which, however, is the uniform 

 ciliary clotliing of the under surface characteristic of Chilodon cucidlulus. No illus- 

 tration of this interesting form, which has so far been observed by Stein only, 

 has as yet been published. 



Fam. V. OXYTRICHIDA, Ehr. 



Animalcules free-swimming, ovate or elongate ; usually with a flattened 

 or concave ventral and a more or less convex dorsal surface ; peristome- 

 field ventral, triangular or arcuate ; oral ciliary system consisting of an outer 

 or right-hand marginal fringe of powerful adoral cirri, which is frequently 

 supplemented by an oppositely reflected or left-hand marginal series of 

 smaller pre-oral cilia, and more rarely by a median series of lax and 

 attenuate endoral cilia ; locomotive cilia setose, stylate, or uncinate, 

 variously distributed upon the remaining ventral surface and forming 

 separate sets or groups, distinguishable as the frontal, ventral, anal, 

 marginal, and caudal series ; supplementary, immolile, hispid, or hair-like 



