REPORT ON THE CRINOIDEA. 177 
very different from that of Marsupiocrinus, and is more like that of the Actinocrinoid ; 
but the apical dome plates (orocentral and orals) take up the greater part of the summit, 
so that the radial dome plates are but little developed, and do not follow the arrangement 
of the calyx plates, as described by Wachsmuth in Strotocrinus and other Actino- 
crinide. Those of Platycrinus extend on to the free rays as a double row of alternating 
plates. Wachsmuth says nothing about the covering of the arms; but there are two 
rows of alternating plates on the pinnules, and these are obviously covering plates. It 
is difficult to believe that the food-grooves of the arms were unprotected, when those of 
the pinnules borne by them were bordered by covering platés ; and if these were present 
on the arms, what was their relation to the two alternating rows of radial dome plates 
upon the free rays? Should not the alternating plates of the free rays, arms, and 
pinnules be considered as parts of one system, just as the small covering plates of the 
pinnules of Pentacrinide are traceable into the larger ones on the arms, and through 
them into those of the disk (PL XVII. fig. 7; Pi) MX figs, 3574; 6; Pl, XGE 
figs. 4, 13; Pl. LIV. fig. 7)? 
The free rays of Platycrinus find a parallel in many recent Crinoids. In Platyerinus 
burlingtonensis the first division porne on the radial axillary consists of two joints only, 
of which the second is axillary. “One face of this bears an arm directly, while the other 
supports two pieces in direct succession, the second of which is an axillary piece 
and gives origin to two amms.”! This arrangement would be described in a Pentacrinus 
or Comatula as consisting of two distichal and two palmar joints, the latter only 
occurring on the inner pair of every two secondary arms; so that the arm formula 
would be 1, 2, 2; 1, instead Ob OV 1 WO sas an Pentacrinus alternicirrus, &e. 
(Pl. XXV.). Many Comatulidee and Pentacrinide have more than two joints in 
the distichal and palmar series, and these do not belong to the arms proper, 
but support radiating extensions of the disk, which is often much incised and 
heavily plated (Pl. L. fig. 2; Pl. LV. figs. 3-7). In the dry state the plates 
remain attached to the distichal and palmar joints, and cover them in by a kind of vault 
(Pl. XXXUIL. fig. 6), just as the radial dome plates cover in the free rays of Platycrinus, 
though on a much smaller scale. Asin the case of Cyathocrinus and Marsupiocrinus, 
therefore, I believe that these alternating radial dome plates of Platycrinus are really 
covering plates of the ambulacra, though permanently closed down, and terminating 
against the primary radial plates of the dome outside the ring of proximals. 
These primary radials are sometimes well developed, as in Talarocrinus and 
Pterotocrinus. In the former’ “ the first radial vault-piece is spiniferous in most species, 
the succeeding plates small and nodose, arranged longitudinally in rows, forming together 
regular arches over the ambulacral passages within the body.” The vault of Pterotocrinus 
seems to have had a closer resemblance to that of Actinocrinus than is the case in most 
1 Paleontology of Illinois, vol. v. p. 453, pl. iii. fig. 6. 2 Revision, part ii. p. 86. 
(zoOL, CHALL. BXP.—PART XXXIL—1884.) Ti 23 
