Gatty Marine Laboratory, St. Andrews. 185 
cylindrical to the curvature at the tip, after which it tapers 
to a fine point. Serrated wings commence a little below 
the curvature, widen, and then diminish distally. Bristles 
appear to be absent in the anterior part of the posterior 
region, only about twelve to fifteen of the terminal segments 
having a pair of bristles on each side. The more anterior of 
these have short straight shafts and sickle-shaped tips, the 
broad tapering blade having its edge serrated. Toward the 
tip of the tail, however, the terminal blade is knife-shaped 
and less curved, a slight enlargement occurring at the end 
of the shaft. Faint striz in all cross the tip obliquely from 
the serrations. : 
The anterior hooks are very diaphanous, with a minutely 
serrated anterior margin ending inferiorly in a slightly pro- 
jecting and modified (blunt) main fang, below which a gulf 
occurs above the prow. The crown is rounded, and the 
posterior outline has a deep indentation. The body of 
the hook is faintly striated from the anterior face to the 
base. The posterior hooks do not differ except in size. 
The collar in Ditrypa arietina, O. F. Miiller, the 
eighth species, though deep, is very thin and its edges 
laciniated, whilst its surface is marked by the linear 
streaks caused by the adpressed branchie. It is fissured 
in the mid-dorsal line, but appears to be continuous from 
side to side across the ventral surface. No alar membrane 
is visible dorsally, though a trace of it probably exists. 
The branchiz are of considerable length, arranged in two 
semicircles of about a dozen filaments in each, and appear 
to adhere closely together, as if they were bound by a deli- 
cate web, though this has not been clearly made out except 
at the base. The filaments taper from base to apex, and 
end in a short process which scarcely projects further than 
the adjoining pinne, though it is twice as thick and presents 
amore distinct central space, whilst its surface has cilia. 
The pinne are long, in a double row, and are so arranged 
distally that they form a nearly even series, and thus give a 
character to the tip of the branchie. Like the filament, 
they possess no skeletogenous elements, though in the 
former the cuticle is thick and tough. The free terminal 
process of the filaments may have special branchial func- 
tions when the animal withdraws into its tube, for they 
project all round in the space below the opercular plug. 
The pedicle of the operculum is long, and springs froin 
the dorsal edge of the left branchial fan, and it remains 
nearly cylindrical to the tips of the branchizw, where it 
