MORPHOLOGICAL PART. 113 
latter were true vault plates, as Carpenter inferred we did. We held that 
while in the vault of the typical Actinocrinus the interradial dome plates 
meet over the ambulacra, in Platycrinus these plates opened out so as to 
expose the covering pieces, and that these were gradually incorporated 
into the vault. In a typical Platycrinoid, the covering pieces are so 
modified as to lose almost their original character, being as large and 
nearly as heavy as the surrounding plates, and they are united with one 
another, as well as with the latter, by close suture. In some of the later 
Platycrinide they even may have been separated from the food grooves, 
for in the internal casts they left no impressions ; while in casts of Actino- 
erinus from the same localities, and in casts of certain Silurian Platycrinide, 
the outlines of the ambulacra are generally sharply delineated (Plate LXXV. 
Fig. 14). 
Carpenter probably supposed the ventral structure of the Melocrinids 
and Rhodocrinidz to be in the same condition as that of the Actinocrinidse 
and Platyerinide respectively, that is, a disk when the ambulacra are ex- 
posed, a vault when they are concealed. He alluded to Glyptocrinus in 
connection with the Reteocrinids and Ichthyocrinide, in all of which the 
ventral pavement is composed of an immense number of very minute, 
irregularly arranged pieces. In the Ichthyocrinide these plates are tray- 
ersed by regular rows of alternating pieces, passing out from the oral centre 
to the arms; in the two other families, however, such alternating plates, 
when present.at all in the tegmen, are found only close to the arm bases. 
Carpenter says respecting these groups,* “‘ I venture to think that in the case 
of Glyptocrinus, Reteocrinus, Xenocrinus, and also of the Ichthyocrinide, the re- 
semblance to the Pentacrinidz, Apiocrinide, and Comatule, is such as to 
leave no reasonable doubt that the so-called vault of these Palaocrinoids is 
homologous with the ventral surface of the body in the Neocrinoids.’ This 
is true enough as to Zurocrinus and Onychocrinus, and probably the Ichthyocri- 
nide generally, in which mouth and food grooves are exposed, as we have 
found out from actual observation ; but in the case of Glyptocrinus and Reteo- 
crinus, there is nothing to prove it beyond a superficial resemblance of the 
plates. Carpenter’s argument loses much of its force, considering that 
among the Actinocrinide within the same genus some species have large 
plates, others very small ones, and the evidence seems rather to prove 
that either these plates are all disk plates, or none of them are. 
* Chall. Rep. Stalk. Crin., p. 185. 
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