of Edinburgh, Session 1876-77. 
409 
like joints. The brachials are also provided with strong lateral 
processes forming a wall on either side of the radial groove, and 
the sides of adjacent series of these first eight arm-joints are 
marked with corresponding grooves and ridges, so that, although 
from the presence of articulating ridges of varying degrees of ob- 
liquity, and of muscular impressions; the proximal portions of the 
arms must be capable of some motion, that motion would appear 
to be slight. After about the eighth the joints suddenly contract 
in size and become greatly compressed, and this narrow teries 
extends to about sixteen in number, gradually tapering to the end 
of the arm. 
At the bases of the arms, just above the edge of the cup, five 
thick calcareous bosses, each composed of the contiguous lateral 
processes of two radial-axillary joints, project interradialiy into the 
cup ; and opposite these five rather large triangular plates, meeting 
in the centre of the disk, form a low pyramid covering the mouth. 
The oral plates are interrad ial, and the spaces between them radial 
corresponding with the arm-grooves. 
D'Orbigny described the animal as possessing no anal opening, 
and this is probably the case, but the material is still too scanty 
to admit of the full examination of a complete specimen of the 
skeleton, and the soft parts are unknown. 
All the specimens of Holopus which have been hitherto pro- 
cured are in a very peculiar condition ; the thick-walled foot, and 
massive, somewhat rudely shaped cup and arm-joints are formed 
of a loose spongy calcified areolar tissue deeply stained with a 
black-green pigment. There is no appearance of any separate 
organic matter, either on the outer surface of the skeleton, which 
is very delicately sculptured like shagreen, or on the articulating 
surfaces of separated plates ; indeed, the whole body is so perfectly 
hard and rigid that at first sight I thought it might be semi-fossil. 
It is without doubt recent, but I suspect that the tissues are very 
imperfectly differentiated, almost protoplasmic. When an arm is 
put into boiling water it falls into pieces at once, the joints simply 
coming asunder, and showing no trace of muscular or other or- 
ganic connection except the axial cords of the joints, which some- 
times keep two joints hanging in connection for a little. 
Holopus is thus specially characterised among living Crinoids 
