180 ECHINODERMATA—PELMATOZOA SUB-KINGDOM III 
the plates forming the side-walls of the calyx, being absent from its upper 
surface. In still other instances (Plewrocystites, Callocystites), (Fig. 297), the 
pore-rhombs are greatly reduced in number, and occur in the form known as 
“ nectinated rhombs.” 'The component halves of the latter stand on contiguous 
plates the same as the ordinary pore-rhombs, but are always separated 
externally by an interval; frequently the two parts are of different form or 
size, and sometimes one of them may become obsolescent. 
As regards the functions of these canals and pores (the ‘“ hydrospires” of 
Billings), the anatomy of existing Crinoids furnishes us with no positive con- 
clusions. They have been compared with the pores which are present in the 
tegmen of the latter, and 
the rather plausible sugges- 
tion has been offered that 
they served to admit water 
into the body-cavity, and 
thus performed respiratory 
functions. At all events, 
they could not have served 
for the protrusion of tube- 
feet, since they are fre- 
quently covered over by an 
Fic. 297. outer calcareous membrane, 
Callocystites Jewetti, Hall. Silurian (Niagara Group); Lockport, : 
New York. A, Calyx from one side (natural size). B, "Ambulacral which effectually shuts off 
gs ou ie eerie gurehs (0), ens; o, Mewth: communication | waa Milam 
exterior. 
The arms in the Cystideans are but feebly developed, and are sometimes 
entirely (?) wanting, or they are but few in number (2, 3, 6, 9-13). The pent- 
amerous symmetry, so generally characteristic of Echinoderms, pervades neither 
the arrangement of the calyx plates nor the number and disposition of the 
arms. The latter are invariably simple, are either uniserial or biserial, and 
exhibit a ventral groove protected by covering plates. 
In some genera the arms attain considerable thickness, but in others they 
are very diminutive, and seem to have closer affinities with pinnulae than with 
the arms of Crinoids. In the Callocystidae and Agelacrinidae, as well as in the 
Canadian genera, Amygdalocystites and Malocystites, the arms either recline with 
their dorsal side facing the calyx, or they are incorporated into the calyx. 
The ventral side, in these cases, is directed outwardly, and the ambulacral 
furrow is bordered on either side by a row of alternating, jointed pinnules, 
which are attached by small articular facets running parallel with the 
groove. Pinnules have not as yet been observed in <Agelacrinus, but the 
allied genera, Mesites and Asteroblastus, are furnished with delicate facets for 
their attachment. 
The stem, as a rule, is greatly abbreviated, and is frequently obsolete. 
Sometimes the calyx is attached by the entire lower surface (Agelacrinus) ; or 
in other cases by means of a tubercular process (Hchinosphaerites). Only in 
rare instances does the stem appear to have served for attachment, since it 
generally tapers distally to a point, and is invariably destitute of cirri. The 
stem sometimes resembles that of the Crinoids, in being composed of a number 
of short, prismatic, or cylindrical joints; these are pierced by a wide canal, 
and are either united by horizontal, striated, articular surfaces, or they over- 

