MORPHOLOGICAL PART. 119 
mouth in the older Crinoids is not accomplished by any additional set of 
plates, but by the plates which in recent Crinoids surround the mouth, 
Cases in which the ambulacra enter the tegmen from beneath what 
were formerly called the interradial dome plates, are found in almost 
every family of the Camerata. They occur more frequently among genera 
in which the ventral surface is paved by small irregular pieces, such as 
Glyptocrinus, Reteocrinus and Archeocrinus ; but also in others. A most 
instructive case of this kind is presented by a rather young specimen of 
Megistocrinus nobilis (Plate XLVII. Figs. 8a, 6), in which not only covering 
pieces, but well defined side pieces enter the calyx. The ventral pavement 
consists of moderately large, irregularly arranged plates, which gradually 
decrease in size toward the arms. The tegmen is perfectly flat, except near 
its outer margin, where it is distinctly plicated to form the large openings 
for the ponderous arms. At the inner flat portions the ambulacra are con- 
cealed, but at the plicated outer part both covering- and side-pieces come to 
the surface, and are visible for some distance. It is now quite instructive 
that in another more adult specimen of the same species (Plate XLVI. Fig. 
6) the parts of the ambulacra which in the former specimen were exposed, 
are roofed over from both sides by interambulacral plates of subsequent 
growth. This observation throws important light upon the development of 
the so-called vault of the Camerata generally. It shows that the same system 
of plates, which in a young specimen is in part inter-ambulacral only, may 
gradually become swpra-ambulacral in another. 
We find a somewhat different structure in a finely preserved adult speci- 
men of Megistocrinus Evansi (Plate XLVII. Fig. 14), in which in three of its 
rays two series of large, nodose, alternating plates pass out from near the 
orals in the direction of the ambulacra. The series are frequently inter. 
ver 
rupted by small, flat pieces, which are interspersed among the larg 
ones. In some places the arrangement of the larger plates, which are 
evidently covering pieces, is as regular as in any Platycrinus ; but at 
others, owing to the interference of the smaller plates, quite irregular, 
especially in the two rays to the right of the anus, where scarcely any 
two of those plates are continuous. It is most remarkable that in no 
two specimens of this species is the arrangement of the covering pieces 
alike. In some of them, only the five large bifurcating plates, the so-called 
radial-dome pieces, are in view, followed by ten others of a second order. 
The ventral structure of this species not only offers a good proof that the 
