EEPOET ON THE ANNELIDA. 
523 
The branchiae in the best specimen measure about 6 mm. in length, and, as removed 
from the tube, present nearly straight radioles. The latter are free, with the exception 
of the narrow basal region, and are supplied with richly ciliated pinnae, which, instead of 
gradually diminishing toward the tip, retain considerable length, so that in profile the 
tip is almost clavate. Each radiole tapers toward the extremity, and ends in a slender, 
smooth, free filament. 
The operculum (PI. LV. fig. 6) extends a little beyond the tips of the branchiae, and 
consists of a somewhat small, circular, whitish plate strengthened by a calcareous deposit, 
and having a yellowish line at its base. It is supported on a hollow dilatation, shaped 
like a wine-glass, the narrow part being attached to the long slender pedicle, which has 
an enlargement at the junction, with several crenations beneath. The entire organ is 
longer than in Placostegus tricuspidatus, Sowerby. No trace of an abortive operculum 
occurs on the left side. 
None of the specimens is sufficiently perfect to show the exact condition of the 
cephalic collar, or the number of the tufts of thoracic bristles. The latter are pale 
yellowish, the dorsal forms having rather elongated tips (PI. XXXa. fig. 25). The 
posterior bristles, again, possess elongated shafts and short extremities with broad wings 
(PI. XXXa. fig. 26), and the curvature at the tip is pronounced. 
The anterior hooks (PI. XXXa. fig. 27) present a shape similar to those of Serpula 
philippensis, and have six weU-marked teeth above the great fang, which projects 
considerably beyond the others. The dorsal outline shows a long concavity. The prow 
below the great tooth is slightly developed, though it is much more so than in Serpula 
philippensis. The body of the hook is crossed by numerous striae, which form the usual 
angle with the teeth. The posterior uncini possess a more elevated crown, and the prow 
beneath the great fang is less developed. 
The dense tube (PI. LV. fig. 5) is fixed to manganese nodules or other similar 
concretions. The attached surface is flattened, the rest is irregularly rounded, 
a deep longitudinal groove running on each side a little above the flattened 
basal region (in transverse section), so as to cut the transverse ridges into isolated 
bars. From this groove the ridges are continued dorsally, and in front often with a 
direction somewhat forward and inward. In the same region of the tube the ridge has 
a slight mucro or thickening, projecting forward in the middle line, the anterior margin 
of the ridge on each side thus presenting a slight concavity. Behind the foregoing the 
transverse ridges are nearly straight, while posteriorly the mucro of the dorsal ridge 
points backward, and the lateral thickening on each side is very decidedly marked. 
Internally the surface of the tube is perfectly smooth and white. 
In transverse section of the body- wall posteriorly, a firm but thin cuticle, and a very 
thin hypoderm are visible. It is difficult to distinguish the circular muscular coat as a 
separate layer. The longitudinal dorsal muscles form long plates, which extend upward 
