248 Rofe—Pentremites, &c. 
appears to be a fillet round the aperture, very like that round the 
ovarian openings in Gorgonocephalus, excepting that in Rhodo- 
erinus the fillet is formed by a small plate, as shown in fig. 6, and 
not by a thickening of the integument. 
Mr. Billings, in the ‘Third Decade of the Geological Survey of 
Canada,’ in treating of the ambulacral openings of Rhodocrinus, 
says—‘ In no other species is there more unequivocal evidence of the 
existence of these openings; but they are accompanied by a struc- 
ture which seems to indicate two sets of arms placed one above the 
other. Beneath the orifices there are two articular surfaces, which 
mark the bases of two arms; and above each pair of the orifices 
there is a projection, which also much resembles the base of one or 
two more arms. . . . Insome of the specimens this feature is exhi- 
bited so prominently that it strongly impresses the observer with the 
idea of two tiers of brachial appendages. It may be, however, that 
there were projecting from this part of the vault a set of large spines 
corresponding in numbers with the arms.’ Since, however, Mr. Bil- 
lings’s paper was published, more perfect specimens have been 
found with portions of the arms attached; and their articulated 
structure, and the passage through the axis, forbid the idea of their 
being spines; and an examination of the specimens in the Museum 
will leave little doubt of the true nature of either the arms or the 
lower openings. No specimen has yet been found showing distinctly 
the course of the upper passages, as in the other Crinoidea; but 
they evidently turn upwards under the dome. The anal opening is 
excentric; and, supposing the passages to exist, as there is reason 
to suppose they did, as in other genera, this opening would be 
between them. 
4. Pentremites. 
Having so far examined the Crinoidea, it became a matter of in- 
terest to see how far the Blastoidea corresponded with them. As 
in the former case, a number of weathered specimens were procured, 
and sections of more perfect ones were made; and from their exa- 
mination it appears that the external characteristics of Pentremites 
are correctly described by M.de Koninck. The base, like that of 
Platycrinus,* is composed of three pieces; two, of equal size, being 
pentagonal, and the other quadrilateral and smaller. Upon this 
base rest five ‘radial plates,’ similar in form and size. They are 
attached to the base-plates as in Platycrinus; that is, two of these 
plates join to the upper sides of two of the pentagonal pieces ; whilst 
the other three alternate or break joint, and are in contact with two 
adjacent plates of the base. The radial plates are very similar to 
* In well-preserved specimens of plates of the Crinoidea, and probably of all the 
Echinodermata, the face of the joints of the plates is minutely wrinkled or corru- 
gated, which will at once distinguish true joints from fractures. By this means the 
doubt as to the construction of the base of the Platycrinus has been removed, a 
divided base having been found showing the wrinkled structure on the face of the 
joints. As these bases are generally found whole, and very rarely in detached: 
"plates, most probably M. de Koninck’s view, that in full-grown specimens they are 
anchylosed into one, is correct. 
