76 
THE YOYAGE OF H.M.S, CHALLENGER. 
minute, about 0’024 mm. in diameter, and irregularly formed. Towards the extremities 
of the pedicels the deposits are more asymmetrical and sometimes rod-like. 
There is no doubt that this species is nearly allied to Semper’s Ocnus imhricatus, and 
mainly differs from it by the possession of buttons, and by the fact that the scales do 
not imbricate. Both forms seem to be very typical representatives of the genus Ocnus, 
and are characterised by a hard, thick perisome containing scales, as well as other deposits, 
and by the arrangement of the pedicels in distinct simple rows. 
Genus Colochirus, Troschel, 1846. 
Colochirus spinosus (Quoy and Gaimard), 1833 (PL XIV. figs. 3, 4 ; PI. VI. fig. 12). 
Habitat. — Port Jackson (Australia) ; depth, 6 to 7 fathoms ; fifteen to eighteen 
specimens. 
The largest specimen attains a length of 80 mm. The body is oval or elongate, 
more tapered posteriorly than anteriorly. In most of the specimens the mouth, and espe- 
cially the anus, are turned upwards, the body acquiring thus the appearance of an Acidian. 
The mouth is closed by five projections of the perisome, and the anus is surrounded 
by about five conical teeth, which in most cases are difficult to discover because they 
have their place within the anal aperture. The anal portion of the body is covered with 
imbricating scales, which decrease in size towards the aperture itself. Excepting a simple 
row of six to eight sharp, conical prominences along each side of the body, the 
surface of the animals is almost smooth, but presents numerous small pits from the 
completely retractile pedicels and “ papillae.” The two ventral tentacles are much smaller 
than the remaining eight. A single madreporic canal and Polian vesicle are present. 
The ventral pieces of the calcareous ring are narrower than the rest. 
The ambulacral appendages are very minute and retracted in all the specimens, so that 
it is almost impossible to decide whether two kinds are really present. They are scattered 
without order all over the body, but are a little more crowded on the ventral than on 
the dorsal surface ; only along the odd ambulacrum is a double row of ambulacral 
appendages traceable. As far as I can find, the only difference existing between the 
dorsal and ventral appendages is that the latter possibly has the sucking-disk slightly 
larger, and the terminal plate very slightly more developed, but both of them are 
strengthened by supporting rods of about the same shape. Each of the conical promin- 
ences along the sides of the body is in possession of an ambulacral appendage. 
The calcareous deposits have about the same appearance as those in Colochirus 
inornatus, to which the species in question bears the nearest resemblance. The 
rounded or oval reticulate thick scales never attain a greater size, the largest having 
a diameter of from 2 to 3 mm. The cups are knobbed (PI. VI. fig. 12a), very 
