12 W. G. RIDEWOOD. 
Polypides in separate cavities, each with a single ostium (sub-genus Jdiothecia). 
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., ee tubes longer 
than their width, each with a short lip, no spines. : . _ levinseni. 
Width of branch 5 to 10 mm., peristomial tubes short, pines fou: 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. levinsent and C. nigrescens) tends to show that 
Cephalodiscus is more closely related to Rhabdopleura 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 Ahabdopleura,* as in C. levinsent 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 Rhabdopleura 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. Possibly 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, 18, 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 Rhabdoplewra, Lankester’s classical memoir (18) 
has been freely drawn upon, also papers by Allman (Quart. Journ. Micro. Sci., n.s. ix., 1869, pp. 57-63, pl. 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 piles id. Proc, Roy. Soc., lii., 1898, p. 182; id. Quart. Journ. Micro. Sci., n.s. xlviii., 1904, 
pp. 28-31, one plate); Schepotieff (Bergens Mus. “Aestbos; 1904, No. 2, pp. 1-21, three plates ; id. Zool, Anz., xxviii., 
1905, pp. 795-806, seven figures), and Harmer (10, pp. 125-128). An independent examination of Bhabdoploina 
was not made. 
