MORPHOLOGICAL PART. m 



A structure similar to that of Steganocriniis 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 Strotocrinus. 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. 



Eeferring again to Platycrimm, 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 Cyaihocrimis. And he 

 continues : "I do not myself think that the vault of a Flatycrinite was exactly 

 of the same nature as that of an AcUnocrinite, i. 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 tegmen calycis as in Actmocrmiis.'' And in alluding to the teg- 

 men of Marsiipiocrinus, he said : t " I 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 Pentacriniis Wyville-thomsoni or of many Antedons (Plate XYII. 

 Fig. 6 ; Plate LY.) as would lead to the supposition that the homologue of 

 the latter is to be sought for beneath the vault of Marsupiocriniisy 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 Actinocrinidae." That is to 

 say, in short : Vault and disk are entirely distinct structures ; and the 

 ventral surface of one Palaeocrinoid represents a disk because the am- 

 bulacra are external, while that of another is a vault because they are 

 subtegminal. 



* Cliall. Eep. Stalked Crinoids, p. 178. 

 t Chall. Rep. Stalked Crinoids, p. 176. 



