372 PLACOSTEGUS TRIDENTATUS. 
1908. Platostegus tricuspidatus, Ehlers. Deut. Tiefsee-Exped., p. 162. 
1909. sf ie Fauvel. Bull. Inst. Oceanogr., No. 142, p. 69. 
1912. * « Wollebek. Skrift. selsk. Krist., Bd. 11, No. 18, p. 117, pl. xlvui, figs. 
1—8; pl. li, figs. 2 and 3. 
1914. ws + Fauvel. Campag. Scient. Monaco, Fasc. xlvi, p. 351. 
1917. ss . Rioja. Anél. Poliq. Cantab., p. 89. 
Habitat—Dredged twenty-five miles N.N.E. of Unst, Shetland, in 85 fathoms; in 
90, 110 and 125 fathoms 50 miles west of Valentia, Ireland (J. G. J.). It is especially 
abundant in the north and west, such as Lerwick, Aberdeen, the Hebrides, Loch Long (J.G. J.). 
Abroad it ranges from Sweden to Spitzbergen, and Finmark (Lovén, Goés, Ljungman, 
Malmeren and Sars); Norway (Norman, Wollebek); Finmark (Norman); 300 fathoms off 
Norway (Sars); Madeira (Langerhans); Azores (Fauvel); Naples, where it is rare and 
found at 250 metres (Lo Bianco); shores of Cantabria (Rioja). 
The collar in this form is so thin as to be diaphanous, but it 1s deep, and is joimed by the 
alar membrane dorsally on each side of the hiatus, passing thereafter across the ventral 
surface to the other side. It is usually thrown into various frills, but it presents no notch 
or break, though it is easily lacerated. 
About twenty-eight branchial filaments occur in each fan, and they are of considerable 
length, little tapered, and ending in a short subulate process. The pinne are comparatively 
short, but they pass to the base of the terminal process without apparent diminution, so 
that the effect is to widen the tip. No skeletogenous element appears in either filament or 
pinna, but the cuticle of the former is thick. 
The operculum arises by a stout pedicle on the dorsal edge of the left fan (Plate CX XII, 
fig. 3), which is considerably thicker than a branchial filament. The pedicle is flattened 
inferiorly, gradually dilates in its upward course, and then enlarges into the clavate 
operculum, which is truncated and hollowed out, yellowish when seen laterally, somewhat 
olivaceous on its distal surface. In lateral view the ventral outline of the apparatus is the 
more convex, the dorsal being nearly straight. In the preparations it is flexible. 
The body is widest anteriorly, the alar membrane increasing its bulk in this region, then 
tapers a little to the tail with the anus at the tip. It is rounded dorsally, slightly flattened 
ventrally, where a median groove runs throughout the posterior region. 
Six pairs of bristle-bundles occur m the anterior region as in Pomatocerus, and each 
has three fascicles. The pale golden bristles (Plate CXXXI, fig. 8) are a little narrowed 
at the insertion, have straight shafts, and slightly curved tapering tips, which end in trans- 
lucent hair-like points, and with very narrow wings. The posterior bristles are few in number, 
two or three, as a rule, being in each foot. The shaft is slender and nearly cylindrical, but 
is narrowed below the distal enlargement, which forms a flattened blade, with an almost 
transverse spinous distal edge, one angle of which is produced into a short whip or pointed 
process. It is remarkable how closely such a bristle resembles the brush-shaped forms of the 
Kunicidee, of Hydroides, Pomatocerus, and other groups, and approaches in outline also the 
paleole of the Sabellarians, as well as the broadly winged forms seen in Leprea. It is a 
modification of the much bolder series seen in the same region of, for instance, Placostegus 
actinocerus, Grube, from Samoa. In this form the shafts of the tail-bristles are brownish 
and slightly bent, the distal portion narrowing a little before terminating in the broad, boldly 
