120 THE CRINOIDEA CAMERATA OF NORTH AMERICA. 
so-called radial dome plates, as before suggested, are extravagantly develope 
covering pieces, but indicates that the “vault” was formed by a gradual 
extension of the interambulacral areas along the line of the ambulacra, either 
covering them entirely, or encroaching upon them and leaving the more 
prominent plates exposed. 
In Glyptocrinus and Reteocrinus the tegmen is essentially in the same 
condition as in MJegistocrinus nobilis, but no orals are distinguishable, and 
the whole ventral surface, including the median portions, is covered by 
minute granular pieces. The middle part is evenly convex, but toward 
the periphery there are ridges leading to the arm bases, and on top of 
these ridges the covering pieces are exposed. In the median portions the 
ambulacra were evidently subtegminal, being roofed over by superimposed 
inter-ambulacral plates, which form a continuous integument over them. 
A careful study of the different tegmens which are found among the 
various families of the Camerata shows that the ambulacra, as a rule, are 
subtegminal in species with a high dome or bulging at the arm bases, but in 
species with a flat or depressed surface they are generally tegminal, or 
become tegminal before entering the arms. They are exposed in forms like 
“ Actinocrinus” quinquangularis, Habrocrinus ornatus, Marsupiocrinus depressus, 
and Glyptocrinus ornatus, — species with a low disk; but they are necessarily 
subtegminal in genera such as Cactocrinus, Siphonocrinus, ete., in which the 
tegmen is high and conical. The condition of the ambulacra, therefore, 
whether tegminal or subtegminal, does not represent an essential structural 
feature, but is a natural consequence of differences in the form and construe- 
tion of the tegmen in the respective species. 
Now if it is true that a plated integument was formed in Glyptocrinus 
and Megistocrinus by the profuse development of the interambulacral plates, 
and their gradual fusion along the line of the ambulacra over a part of the 
disk, it would seem to follow that the more substantial vaults of Actinocrinus, 
Batocrinus, and Physetocrinus may have originated in a similar way, and that 
the body of these forms also was covered by a single set of plates. This 
seems to be confirmed by a very instructive specimen of Physetocrinus, in 
which the delicate structure at the inner floor is shown in excellent preser- 
vation (Plate V. Fig. 14). The specimen has the great advantage of being 
free from any silicious coating, such as obscured the structure in previ- 
ous specimens of this kind. The outer surface of the tegmen is composed 
of moderately large, smooth pieces, of irregular form, closely fitted together 
