PLUMES OF CEPHALODISCUS. 233 



pairs of plume-axes. Both k and l are ventral views, and 

 the groove between the two rows of pinnules of each plume- 

 axis is directed towards the observer. 



It will be noticed that in J, k, and l the right and left 

 plumes of the first pair are separated by a small interval^ in 

 which is situated a slight mound of the basal part of the 

 stalk of the buccal shield, obliquely torn in the making of 

 such preparations as those here figured ; but the several 

 plumes of the same side of the body are in close contact 

 with one another at their bases, and the last developed plume 

 is wedged in, with a certain amount of over-lapping, between 

 the last plume but one and the antero-lateral edge of the 

 post-oral lamella. 



Except in distorted specimens the last developed plume 

 lies slightly dorsal to the edge of the post-oral lamella, and 

 lies slightly ventral to the last plume but one (k). It must 

 be noted, however, that in stages such as that shown in l, 

 and in later stages, the line of plume bases and base of the 

 post-oral lamella does not lie in a plane, and a certain amount 

 of distortion or crumpling is bound to result from any attempt 

 at mounting the dissection between two pieces of glass. 

 Note the wrinkle in the middle of the posterior edge of the 

 post-oral lamella in l. In such a stage as is shown in J the 

 line of plume-bases and attached edge of the post-oral lamella 

 is nearly in one plane, a plane slightly oblique to the long 

 axis of the bud. The fourth, fifth, and sixth plumes, however, 

 are not developed in this plane, but more dorsally, and this 

 is how it comes about that in the adult polypide the bases of 

 the six plumes are set in a circle or ellipse in one oblique 

 plane of the body, while the attached edge of the post-oral 

 lamella lies in another oblique plane, which intersects the 

 first plane at the points of contact of the sixth plumes with 

 the edge of the post-oral lamella. This will be made clear, 

 I think, by reference to text-fig. 5, and its accompanying 

 legend. See also text-fig. 8, a diagram of Cephalodiscus 

 nigrescens. 



