12 W. G. KIDEWOOD. 



Polyjiides in separate cavities, each with a single ostium (sub-genus Idiothecid). 

 Polypides, black ; plumes, seven pairs. 



Colony massive, width of branch 15 to 25 mm. or more, peristomial tubes very 

 short, each with a short blunt lip . . . . . . . . nigrescens. 



Polypides, not black ; plumes, six pairs. 



Width of branch (including peristomes) about 12 mm., peristomial tubes longer 

 than their width, each with a short lip, no spines ..... levinseni. 



Width of branch 5 to 10 mm., peristomial tubes short, spines long, slender, 

 simple or forked . . . . .an undescribed species from the Cape Seas. 



RELATIONS BETWEEN RHABDOPLEURA AND CEPHALODISCUS. 



The discovery of species of Cephalodiscus with the polypides residing in separate 

 tubular spaces in the tubarium (viz., C. levinseni and C. nigrescens) tends to show that 

 Ceplialodiscus is more closely related to Rhahdopleiira than was supposed ; although 

 it must be borne in mind that the new tubes in the species of Cephalodiscus in 

 question are formed independently of the older ones and not as laterally erupted 

 branches of them (see text-fig. 6). In Rhabdopleura* as in C. levinseni and 

 C nigrescens, the tube is lengthened by additions to its free edge, and, the increments 

 being intermittent in both genera, the successive rings are distinct to the eye, 

 although as a rule not readily separable by dissection. In instituting a comparison 

 between the tubaria of Rhabdopleura and the above species of Cephalodiscus, however, 

 it is to be noted that in the latter the rings are so broad, in a direction at right- 

 angles to or oblique to the axis of the tube, as to form a tube-wall of considerable 

 thickness, or even to constitute strata extending more than half-way towards the 

 adjacent tubes, so that the space which in Rhahdojjleura occurs between neighbouring 

 tubes does not exist. 



In Rhabdopleura each of the rings which compose the tube is in most cases inter- 

 rupted by an oblique suture (text-fig. 7, B; see also Harmer, 10, pp. 8 and 126). 

 The polypide works round the periphery of the tube until it returns to its starting 

 point ; the part of the new ring that was first secreted having by this time hardened 

 somewhat, the junction of the first-formed and last-formed parts of the ring is indicated 

 by a suture. Possil:)ly in some cases the secretion of the ring is so rapid that the suture 

 is not discernible, while in other cases the polypide — usually an immature polypide 

 with a bilobed buccal shield — forms only a half-ring at a time, so that the complete 

 ring has two sutures (Lankester, 13, p. 627). 



In Cephalodiscus levinseni and C. nigrescens the rings that constitute the tubes are 



* In these remarks upon the structure and mode of growth of Bhabdopleura, Lankester's classical niemoh' (13) 

 has been freely drawn upon, also papers by Alknan (Quart. Joum. Micro. Sci., n.s. ix., 1869, pp. 57-63, pi. 8), Sars 

 (Quart. Journ. Micro. Sci., n.s. xiv., 1874, pp. 23-24, one plate), Fowler (Festschr. 70ten Geburtstage R. Leuckarts, 

 1892, pp. 293-297, one plate; id. Proc. Roy. Soc, Hi., 1893, p. 132; id. Quart. Jomn. Micro. Sci., n.s. xlviii., 1904, 

 pp. 23-31, one plate) ; Schepotieff (Bergens Mus. Aarbog, 1904, No. 2, pp. 1-21, three plates ; id. Zool. Anz., xx^-iii., 

 1905, pp. 795-806, seven figures), and Harmer (10, pp. 125-128). An independent examination of Bhabdopleura 

 was not made. 



