PLACOSTEGUS TRIDENTATUS. 373 
serrated blade, which then tapers to a fine point (Plate CXX XVII, fig. 18). In the British 
species the distal blade has been reduced to an insignificant size, but the type has been more 
or less retained. The size of the representatives of the Serpulids in the tropical and sub- 
tropical waters is as noteworthy as the proportions of their bristles and hooks. Those of 
our seas in both respects are diminutive. 
The anterior hooks (Plate CX XXI, fig. 8a) are in single rows, very numerous and fairly 
large, but it is difficult from their translucency to define their exact outline. The anterior 
edge is nearly straight and saw-like from minute denticulations, the last of which (probably 
corresponding to the main fang) projects outward and downward as a minute blunt process, 
the body of the hook forming a narrow, flattened and transparent thin plate. The crown 
is small and rounded, and the whole body of the hook is easily curved under pressure. 
The posterior hooks do not differ essentially in structure or arrangement though they are 
smaller. 
Reproduction.—An example procured off North Unst by Dr. Gwyn Jeffreys in July had 
a series of small ova in a hollow on the ventral surface behind the anterior region, so that 
it would appear to develop its eggs in the tube. 
The tube is of great density, slightly translucent or vitreous, and has anteriorly a sharp 
dorsal spike and two infero-lateral spikes. A serrated keel runs along the mid-dorsal line, 
and the tube is fixed to shells, stones or other submarine bodies. The inner surface is smooth 
and vitreous, and the minute teeth along the anterior edge of the hook are of great 
hardness. Whilst the initial coil is adherent the distal end stands up freely in many cases. 
When densely grouped on stones the tubes are less rough, and the aperture is smoother, 
and as they are coiled and interwoven, and their ends often free, a considerable change thus 
takes place in the facies of the tube. It still retains its median ridge, but in a modified 
form. The whole aspect of the mass is in contrast with the ordinary condition of the species. 
Yet here and there at the edges the usual aspect is observed. The examples indicated came 
from the deeper water off St. Andrews Bay attached to sandstone and accompanied by the 
tubes of Sabellaria. 
It is an interesting fact in connection with Arnold Watson’s sketches that Delle Chiaje 
(1822) observed the cameration of the tubes of Serpulids, e.g., in his Serpula triquetroides.' 
He shows a section of the tube with the space at each side. Philippi thought that this 
author’s’ Serpula fimbriata pertained to the present genus. 
Philippi (1844) noticed that the branchiz had two brown bands, and that there were 
nine on each side. It lives at great depths. 
The Placostegus tricuspidatus of Langerhans, S. tricuspidata of Sowerby, S. crystallina, 
Scacchi, and Pl. crystallinus, Philippi, appear to be closely allied, though the branchiz are 
fewer (nine to eleven), and on segment 1 is a band of simple eyes on each side. 
A single example of a variety procured in Shetland by Dr. Gwyn Jeffreys in 1867 
(which, from its size, may be a young form) differs from Placostequs tridentatus in the shape 
of the operculum (Plate CX XXIII, fig. 4), which has the outline of a vase, with a thin yellowish- 
green projecting rim, a constriction following an enlargement, then tapering below to a 
stout pedicle which gradually diminishes from above downward, and thus diverges from 
1 «Memorie,’ tav. Ixvii, fig. 16. 
* Thid., tav. xlvii, figs. 19 and 20. 
