MORPHOLOGICAL PART. 121 
at all sides. There are no orals, but near its outer margin there are radial 
dome plates of a first and second order, which are readily recognized by 
their large size and greater convexity. Other ambulacral plates are not 
visible. Examining the inner floor, we find the same arrangement of plates, 
and actually the same plates as at the outer side, but the general aspect is 
totally different. They look like sharply delineated stars, with as many rays 
as there are sides to the plates. There are abrupt depressions between the 
star-rays, which on meeting corresponding depressions of adjoining plates, 
form deep, sometimes cavernous pits, more or less undermining the plates, 
and which seem to have communicated with one another by imbedded pas- 
sages all along the tegmen. The star-shaped plates extend over both the peri- 
stome and ambulacra, being occasionally interrupted by small, supplementary 
pieces, apparently solid. Thus the tegmen of Physetocrinus is not composed 
of two distinct sets of superimposed plates, as heretofore supposed, but of one 
set only, of which the plates are solid externally, and perforated or honey- 
combed throughout their inner portions. The presence of but one set is 
further confirmed by the position of the ambulacra, which follow the inner 
floor. This is of importance, for if the upper or solid part, as was supposed 
to be the case in the allied Batocrinus and Actinocrinus, represented a vault, 
and the inner part a disk, the ambulacra should lie between them; whereas 
in this case, lying below them both, they would be covered by two integu- 
ments, first by the overlapping interambulacral plates, and then by a vault, 
—an arrangement in the highest degree improbable. The ambulacral skele- 
ton itself is not preserved in the specimen, but the place it occupied is clearly 
indicated by shallow grooves, formed by a thickening of the plates all along 
the interradial spaces. 
The internal structure of Physetocrinus gives us the key to that of Bato- 
erinus, Dorycrinus, Actinocrinus, Teleiocrinus, and of the Camerata generally, 
in all of which, as we no longer doubt, there is but a single integument ; 
and the part which we have heretofore regarded as a disk forms a portion 
of the same set of plates, which are perforated at the inner floor so as to 
produce the numerous caverns and passages above described. These pas- 
sages and pits may have served for the free circulation of water, and we 
think it highly probable that the older Crinoids had a very complex vascular 
water system, extending all the way from the interradial plates of the dorsal 
cup to the top of the ventral disk. This complex system was probably 
not required in recent Crinoids, in which many of the plates are perforated 
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