MORPHOLOGICAL PART. 101 
orals, and when these are wanting, by some of the interambulacral plates ; 
or by both, in which case only small portions of them are seen near the arm 
bases. In the Fistulata, the disk ambulacra are either altogether tegminal, 
or their ends are covered by the orals. In the Ichthyocrinide, so far as 
observed, and in recent Crinoids, they extend to the mouth, whether orals 
are represented or not; but while in Zuzocrinus (Plate III. Fig. 11) they 
are in the same plane with the orals, and are attached to them laterally, 
in recent forms, in which the orals are opened out, they are deeply in- 
serted between the interambulacral plates, so as to be almost obscured. 
The disk ambulacra of the Camerata, if tegminal, form a component part 
of the tegmen, being suturally connected with the interambulacral plates, and 
with the orals. In the Cyathocrinide, however, and probably in other Fis- 
tulata, they rest upon large interradial plates, and between the small margi- 
nal pieces which cover the surface of the latter. In the Ichthyocrinide and 
recent Crinoids, they are separated by minute interambulacral pieces. 
The ambulacra of the Camerata rarely have any side pieces, these being 
represented, so far as known, only in Megistocrinus (Plate XLVII. Figs. 
7 and 8a, 6), in Cactocrinus (Plate LVIIIL. Figs. 7 a, 6), and in Lyriverinus 
(Plate XI. Fig. 4c). They are present, however, in most Fistulata, but 
absent in the Larviformia. 
That the covering pieces in the disk of Cyathocrinus, as suggested by 
several writers, were movable, so as to expose the food grooves, seems to 
us improbable, although there is no serious objection to it from a morpholo- 
gical point of view ; but the perfect preservation of the plates in so many of 
our specimens seems rather to indicate that they were rigid. They may 
have been movable in groups in which the mouth is opened out, but where 
it is closed they were probably rigid throughout the disk. 
In some of the Camerata in which the primary arms are developed into 
tubular appendages, and secondary arms are given off at the sides, as in 
Eucladocrins (Plate LX XII. Fig. 3, Plate LXXIV. Fig. 4), and Steganoerinus 
(Plate LXI. Fig. 1e), the covering plates of the main arms are almost rigid 
to the full length of the ray, and only those of the side arms and their pin- 
nules were movable. But it must be remembered that these appendages are 
practically extensions of the calyx. 
Subtegminal ambulacra, so far as we know, occur only among the 
Camerata and Larviformia. In the former there are frequently along the 
inner floor of the tegmen deep grooves or ducts, which are formed either by 
