370 PLACOSTEGUS TRIDENTATUS. 
and registered (66. 1. 30. 33) ‘‘ Falmouth, W. P. Cocks.” One has a vase-shaped operculum, 
smooth all round and slightly hollowed. ‘The stalk has lateral processes. From the shape 
of the hooks it would appear to be a variety of Pomatocerus triqueter. ‘The same author’s 
example of Serpula Dysteri in the British Museum is P. triqueter with a modified operculum 
developing two spikes on the top. The specimens came from Berwick Bay and Polperro. Dr. 
Johnston’s Serpula conica, which has a conical process developed on the cup-like base, is 
also Pomatocerus triqueter, and the same holds for his Serpula armata, with a bifid crown to 
the operculum, and for Serpula Dysteri. 
Drasche! (1884) described the development of Pomatocerus triqueter from the early stages 
of the segmenting reddish violet eggs to the pelagic post-larval form with three pairs 
of bristle-bundles, and the appearance of the hooks. In the younger form of three days, 
the anal sac, the excretory canal and the mesoderm cells were clearly defined. 
De St. Joseph (1894) observed that the seven segments of the thorax are constant, and 
so with the presence of a small bifurcate tongue or process at the base of two of the first 
dorsal branchiee. 
Genus CLXXXI.—Puacostecus, Philippi, 1844. 
Cephalic collar thin but deep, continuous, except at the dorsal hiatus, where it fuses 
with the alar membrane. Branchize of moderate length, with subulate terminal processes. 
Operculum arising from the dorsal filament of the left fan, with a horny hollow saucer-shaped 
cap distally. Anterior region of the body has six bristled segments. Anterior bristles 
simple, with slightly curved, tapering tips and narrow rings. In the posterior bristles the 
narrow neck expands into a flattened shoulder with the distal edge serrated and a short, 
tapering whip at one border. Anterior hooks with a long and minutely denticulate anterior 
edge, terminated inferiorly by a blunt process (representing the main fang). Posterior hooks 
similar but smaller. Tube fixed, vitreous, translucent, of great hardness, only curved, 
not coiled; the aperture with a dorsal spike and two infero-lateral spikes. 
Philippi (1844) instituted this genus for forms with a calcareous operculum, forming a 
shallow disc, margin entire, and resembling that of a gastropod. He did not refer in the 
generic characters to the nature of the tube. In certain respects his account of the operculum 
of Eupomatus would suit Placostegus as here interpreted. 
1, PLACOSTEGUS TRIDENTATUS, J. C. Fabricius, 1799. Plate CXXII, figs. 3 and 3a— 
operculum and branchia; Plate CXXXI, figs. 8 and 8a—bristle and hook. 
Specific Characters—Collar deep, but thin and translucent, continuous, except at the 
dorsal hiatus, where it fuses with the alar membrane. Branchiz of considerable length, 
filaments little tapered, and ending in short subulate processes. Pinnee rather short, continued 
to the base of the terminal process without evident diminution. Operculum springing from 
the left fan, with a horny, hollow cap of an olive hue.? Body widest anteriorly, tapering to 
1 «Entwickl. der Polycheten, Erstes Heft, Wien, 1884. 
* Langerhans, after Hansen, states that the first segment has groups ef eyes on each side and 
no bristles, and that the operculum has a slit. In the preparations no eyes are visible. 
