MORPHOLOGICAL PART. alata! 
A structure similar to that of Steganocrinus is found in all Camerata in 
which the arms are free from the distichals up ; and from this condition all 
gradations can be traced to the complex structure of Sérofocrinus. The plates 
of the dorsal cup and those of the tegmen are undoubtedly parts of one ele- 
ment; and although they are introduced respectively at the actinal and 
abactinal sides of the calyx, the plates of the one side cannot be regarded 
as strictly abactinal, nor those of the other as actinal. 
Referring again to Platycrinus, Carpenter * said that the series of inter- 
radials found in the peripheral portion of the vault (tegmen)— by which he 
meant the zone between the proximal dome plates (summit plates) in the 
centre and the calyx interradials (of the dorsal cup)— belong to the same sys- 
tem of interradial plates as the single large interradial in Oyathocrinus. And he 
continues: “I do not myself think that the vault of a Platyerinite was exactly 
of the same nature as that of an Actinocrinite, 7. e. that it covered in the whole 
of the visceral mass and the ambulacra on its upper surface. For if the 
alternating dome plates represent the covering plates of recent Crinoids, as 
Wachsmuth suggests, then all the periphery of the dome, outside the apical 
dome plates (orocentral and orals), must be the real ventral surface of the 
body, and not a ¢egmen calycis as in Actinocrinus.’ And in alluding to the teg- 
men of Mursupiocrinus, he said: “1 have a very strong impression that the 
so-called vault of this genus is really the strongly plated ventral perisome, in 
the centre of which the remains of the orals (apical dome plates) are perhaps 
to be found. I cannot see any such essential difference between it and the 
plated disk of Pentacrinus Wyville-thomsoni or of many Antedons (Plate XVII. 
Fie. 6; Plate LV.) as would lead to the supposition that the homologue of 
the latter is to be sought for beneath the vault of Marsupicerinus.” He then 
alluded to the closure of the mouth, and to the covering pieces of the ambu- 
lacra, which may have been immovably closed down over the food grooves, 
saying: “ They were thus converted into tunnels, but were still ‘external,’ in 
the sense of not being covered by a ‘tegmen,’ as those were which formed 
the tubular skeleton beneath the vault of the Actinocrinide.”’ That is to 
say, in short: Vault and disk are entirely distinct structures; and the 
ventral surface of one Paleocrinoid represents a disk because the am- 
bulacra are external, while that of another is,a vault because they are 
subtegminal. 
* Chall. Rep. Stalked Crinoids, p. 178. 
+ Chall. Rep. Stalked Crinoids, p. 176. 
