180 THE VOYAGE OF H.M.S. CHALLENGER. 
plates were more massive than in recent Crinoids, just as the former were in the Cyatho- 
crinide ; and I do not think therefore that there was anything within the vault of 
Platycrinide like the tubular skeleton and the network of anambulacral plates that 
occur in Actinocrinus. The greater part of the above argument appears to me to be a 
mere logical deduction from Wachsmuth’s very suggestive remark’ that the alternate 
plates in the dome of Platycrinide are represented in the recent Crinoids “by the 
‘Saumplittchen,’ which, however, instead of forming a part of a solid vault, are movable, 
and line the lateral margins of the tentacle furrows.” 
Although believing that the vault of a Platyerinoid corresponds collectively to the orals, 
interradials, ambulacral, and anambulacral plates of Neocrinoids, I do not wish to assert 
that the Platycrinidze either had an external mouth or open ambulacra on the disk. For 
I imagine that both were closed as in Cyathocrinus, the whole system of plates being 
much more substantial than in Neocrinoids, and forming part of a solid covering, but not 
a true vault or tegymen calycis. 
In the Actinocrinidz, on the other hand, not only the food-grooves themselves, but 
also their skeleton of alternating plates, were subtegminal, together of course, with the 
plated interpalmar areas of the disk. The oral or actinal system of plates does not 
consist merely of an orocentral with one or two rings of plates round it, which cover in 
the peristome and the origins of the ambulacra from it. It is so greatly developed as to 
cover in and conceal the whole ventral surface of the body, @.e., the disk proper. The 
subtegminal food-grooves passed outwards from the peristome over this upper surface of 
the disk, and were continued on to the arms through the ambulacral openings round the 
dome. A primary dome-radial is always present beyond the orals, and may be followed 
by secondary, tertiary plates, &c. Stelidiocrinus has very few dome-radials, but in other 
types the number becomes very large, in correspondence with the development of 
different orders of radials in the calyx. Sooner or later, however, the subtegminal food- 
grooves reached the arm-openings, and the minute plates protecting them were continuous 
with the skeleton of the brachial ambulacra. 
There is one form which is placed by Wachsmuth and Springer among the Actino- 
crinidz, but has a vault of very different construction from that of the other members 
of this family. In fact it somewhat resembles that of Marswpiocrinus. I mean 
Carpocrinus ornatus (Habrocrinus, Angelin). Wachsmuth describes its radial portions 
as “covered by two rows of low transversed pieces ; interpalmar fields paved by somewhat 
larger and elongate plates.”’ As pointed out above, the word “interpalmar” denotes 
the areas between the radiating food-grooves; and its use by Wachsmuth is therefore 
significant. I cannot resist the suspicion that the double row of low transversed pieces 
indicates the position of a food-groove ; and that the covering plates may have been 
permanently closed down so as to convert the grooves into tunnels, without the additional 
1 Revision, part ii. p. 30. 2 Ihid., part ii. p. 106. 
