REPORT ON THE CRINOIDEA. 181 
help of a tegmen, such as occurs in the Actinocrinide. The proximal ends of these 
tunnels would open into the closed peristomial space beneath the pyramid of apical dome 
plates or orals. In fact, Carpocrinus appears to me to be in the condition of a Hyocrinus, 
with an oral pyramid composed of somewhat smaller plates, but permanently closed, like 
the ambulacra of the disk. While therefore ] am in complete-accordance with Wachsmuth 
respecting the closure of the peristome and calyx-ambulacra in the Cyathocrinide, 
Platycrinide, and Actinocrinide, I cannot altogether agree with him in denying all 
homology between the solid vault of a Palzeocrinoid and the soft disk of a recent form. 
For I believe that both in the Cyathocrinide and in the Platycrinide the plates which 
form the vault are unusually massive representatives of the ambulacral, anambulacral, 
and interradial plates which are developed in the perisome of a Pentacrinus or 
Comatula. 
The Ichthyocrinide and some of the doubtful Silurian forms, such as Reteoerinus and 
Xenocrinus, appear to me to occupy an intermediate position between the heavily vaulted 
Platyerinidee and the more thinly plated recent forms. Some of the Mesozoic species, 
such as Hxtracrinus, Apiocrinus, Guettardicrinus, and Marsupites, which have com- 
paratively thick plates on the sides and surface of the disk, also help to fill up the gap. 
The only genus of Ichthyocrinide in which the summit is known at all satisfactorily 
is Onychocrinus. Wachsmuth and Springer describe it as follows :'—“ Interradials three 
to twenty, perhaps more in some species; the first one large, resting between the first 
and second radials ; the succeeding ones smaller, rapidly decreasing in size and thickness 
upward, and having an inward curvature. They are followed by very minute, irregular 
polygonal plates, which form the interradial portion of the vault. The radial summit 
areas consist of two rows of somewhat larger plates, alternately arranged, which extend 
to the ventral covering of the free rays, and probably throughout their full length. In 
the median portion of the vault there are six rather thin but large apical dome plates.” 
I understand, however, from Mr. Wachsmuth that he is now less inclined to believe in 
the presence of apical dome plates in the Ichthyocrinide ; and I will not therefore take 
their presence as established. If they exist I should call them oral plates, and compare 
the vault to the disk of a Hyocrinus with a closed oral pyramid. But in their absence 
the vault appears to me so closely to resemble the disk of Pentacrinus and Comatula, 
that I cannot question the identity of the two for the merely a priori reason of the 
Ichthyocrinidee being Paleeocrinoids. 
The two rows of alternating plates which radiate outwards over the “squamous 
integument,” and extend on to the free rays (7.e., distichal and palmar series), are surely 
nothing more than the covering plates of the ambulacra, which were perhaps permanently 
closed as in the Platycrinide, or only temporarily so, as in the Neocrinoids; while the 
small irregular plates which form the interradial portion of the vault, correspond to the 
1 Revision, part i. pp. 53, 54. 
