180 THE VOYAGE OF H.M.S. CHALLENGER. 



plates were more massive than in recent Crinoids, just as the former were in the Cyatho- 

 crinidse ; and I do not think therefore that there was anything within the vault of 

 Platycrinidae 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 Platycrinidge are represented in the recent Crinoids " by the 

 ' Saumpliittchen,' 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 Platycrinoid corresponds collectively to the orals, 

 interradials, ambulacral, and anambulacral plates of Neocrinoids, I do not wish to assert 

 that the Platycrinidse either had an external mouth or open ambulacra on the disk. For 

 I imagine that both were closed as in Cyatlwcrimts, 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 tegmen calycis. 



In the Actinocrinidae, 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, i.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- 

 crinidaj, but has a vault of very different construction from that of the other members 

 of this family. In fact it somewhat resembles that of Marsupiocrvmis. I mean 

 Carpocrinns ornatus {Habrocrimis, 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. ^ Ibid., part ii. p. 106. 



