246 Rofe—Actinocrinus, &c. 
Amphoracrinus, showing the passages within the thickness of the 
plates; and fig. 2 is an elevation or side-view of another specimen, 
exhibiting both the openings into the visceral cavity at a, and the 
passages towards the apex at 6. Fig. 3 is a view of the cast of a 
dome of Actinocrinus tessellatus, showing the five passages meeting 
at the apex, with the base of the proboscis in the anal angle. The 
position of the proboscis may be verified by tracing from the anal 
plate at the base. 
2. Cyathocrinus. 
Casts of some species of Platycrinus have been femme showing 
passages similar to those in Aetinocrinus; but in the genus. Cyatho- 
crinus the passages into the viscera and to the apex are better seen 
than in any other; and from this genus a clue to their nature may 
probably be gained. The arm-bearing plates, or ‘ primary radials,’ 
of Cyathocrinus are deeply notched, or cut down, at the insertion of 
the arms; and the upper part of the plate on each side of the arm 
is in-arched more or less in different species, so as partially to cover 
the dome.* The ‘radial plates’ are attached to each other at their 
sides; and at the junction of their upper ends each pair form a very 
obtuse angle, into which a tetragonal plate is fitted; but the sides 
of these last plates are produced across the notch in the ‘radial,’ 
so as to meet each other, the projecting part being grooved out so as 
to form a channel from the base of the arm to the centre of the 
dome, where the five channels meet in a round aperture, which is 
described by M. de Koninck as the mouth ; the proboscis at the side 
being considered the anus. This arrangement of the plates in 
C. planus is shown by fig.4. Ihave not found any radial plates 
formed as described by Messrs. Austin; but in such a case the above 
tetragonal plates would form a continuation of the groove to the 
mouth. Specimens of Cyathocrinus showing the passages from the 
arms meeting at the central opening are not uncommon; but in the 
British Museum, and in Professor Tennant’s collection, there are 
specimens which show that these were covered with thin plates like 
those in Amphoracrinus, thus being within the thickness of the 
dome, as in that genus; and that the mouth, if such was the circular 
aperture, was internal, as in the Ascidians. With the exception of 
the mouth, the only apparent difference in the organization of the 
above Crinoids is that in some the passages are in, and in some 
under, the plates of the dome; and with respect to the mouth, 
although we have as yet no specimen showing its existence in some 
of the genera, it would be too much absolutely to deny it, when in 
so many other respects they appear to be similarly constituted. 
It is probably presumption at present to speculate on the nature 
* Messrs. Austin, when describing Cyathocrinus, say—‘The ray-bearing plates 
are perforated low down from their upper edges, for the passage of the muscles of 
the rays. A deep groove runs from each perforation to the summit of each plate, 
which is prolonged considerably beyond the point of attachment for the rays, and is 
in-arched, so as to partially cover in the vertex. ‘This is particularly conspicuous 
in C. geometricus, and appears to be very common in Crinoids of this genus.’ 
