374 PLACOSTEGUS TRIDENTATUS. 
that of Placostegus from Cape de Gatte, whilst the operculum proper differs in outline from that 
of P. tridentatus. The distal surface of the operculum is horny and concave. The tips of the 
branchie (Plate CX XXI, fig. 9a) show a slender terminal process. The deep, thin collar 
agrees with that of the other forms. Twelve branchiz occur in each of the fans, and the 
filaments have a thick cuticle and end in a subulate process. The pinne are also similar 
to those in the typical form. In the form of the body no noteworthy difference is 
observable, but the two anal papille form two short cylindrical and conspicuous processes. 
The anterior bristles (Plate CXXXI, fig. 9) are typical, and simple delicate tapering 
bristles occur near the tip of the tail as in Placostegus tridentatus. No brush-shaped bristles 
were seen, but they were probably present. 
The anterior and posterior hooks lean rather to those of P. tridentatus, the modified main 
fang being prominent (Plate CXXXI, fig. 9b). No tube is present. On the whole such 
would seem to be a young form of the species. 
Another variety of Placostegus tridentatus was dredged 6 miles from shore off Cape de 
Gatte in 60 to 160 fathoms in the “ Porcupime” Expedition of 1870 attached to small 
stones. It is distinguished from the northern type by the longer and more slender tube 
(Plate CXXXIII, fig. 2), which may be curved here and there but not coiled, and by 
the presence of three coarsely serrate ridges—a dorsal and two lateral. The three spines 
at the aperture also differ in character, since they are longer and curved outward. 
The thin diaphanous collar seems to have the same arrangement as in P. tridentatus, 
but the branchize are proportionally longer, and less numerous, viz., eleven to twelve in 
each fan; the filaments taper a little from base to apex, where a longer terminal subulate 
process than in the northern type occurs. The pinne, however, continue long to its base. 
The opercular stalk is slender, and remains nearly of the same diameter to the base of the 
vase-shaped operculum (Plate CXXXI, fig. 11), which, though smaller, is more elegant in 
shape than in the ordinary form. The truncated distal end is hollow and horny, the rim 
only being yellowish. 
The body is long and narrow, with six bristled segments anteriorly, and numerous posterior 
segments, which are flattened toward the tail, and end im an anus with a distinct and rounded 
papilla at each side. The six setigerous processes anteriorly have tufts of the same kind, 
viz., bristles with straight shafts, very slightly bent tips with narrow wings—in which respects 
they do not materially differ from those of P. tridentatus. Posteriorly bristles seemed to be 
absent until near the tip of the tail, where each segment has a single, long, slender tapering 
bristle or two. The brush-like bristles, however, occur toward the posterior end—in front 
of the simple forms. Hach has a slender, straight stem, which dilates distally into the flattened 
spinous tip—one side of which has a minute whip or poimted process, the tip being thus 
asymmetrical. The anterior hooks appear to be proportionally smaller and broader than in 
the northern form, and the modified main fang is indistinct. In both the serrations of the 
anterior edge resemble transverse grooves of a file, but they are perhaps broader and more 
distinct in the present form. The posterior hooks are slightly less, but agree in structure 
with the foregoing. The tube is small, elongated, tapered posteriorly, and fixed to small 
pebbles. The dorsal and two lateral spinous ridges are diagnostic when compared with 
the tube of the northern form, for the spinous processes on each are isolated and prominent, 
as well as occasionally curved, as are also the three anterior spines. Moreover, the entire 
