REPOET ON THE CRINOIDEA. 177 



very different from that of Marsiqjiocrinus, 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- 

 crinidse. 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 plates ; 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 Pentacrinidae are traceable into the larger ones on the arms, and through 

 them into those of the disk (PI. XVII. fig. 7 ; PL XXXIII. figs. 3, 4, 6 ; PI. XLI. 

 figs. 4, 13 ; PL LIV. fig. 7) ? 



The free rays of Platycrinus find a parallel in many recent Crinoids. In Platycrinus 

 hurlingtonensis the first division borne 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 arms." ^ This arrano-ement would be described in a Pentacritms 

 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 of 2, 1, 1, 2 as in Pentacrinus alternicirrus, &c. 

 (PL XXV.). Many Comatulidaj and Pentacrinidse 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 XXXIII. fig. 6), just as the radial dome plates cover in the free rays of Platycrinus, 

 though on a much smaller scale. As in the case of Cyathocrinus and Marsupiocrinus, 

 therefore, I believe that these alternating radial dome plates of Platycrinus are really 

 covering plates of the amliulacra, 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 Pali-eontology of Illinois, vol. v. p. 453, pi. iii. fig. 6. ^ Bevision, part ii. p. 86. 



(ZOOL. CHALL. EXP. PART XXXII. — 1884.) K 23 



