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 Ichthyocrinidse, so far as 

 observed, and in recent Crinoids, they extend to the mouth, whether orals 

 are represented or not ; but while in Taxocrinus (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 Cyathocrinidae, 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 Ichthyocrinidse 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 Megistocrimis (Plate XLYII. Figs. 

 7 and 8 a, h), in Cadocrimis (Plate LYIIL Figs. 7 a, h), and in Lyriocrinus 

 (Plate XT. Fig. 4 c). They are present, however, in most Fistulata, but 

 absent in the Larviformia. 



That the covering pieces in the disk of Ci/athocrinus, 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 

 Eudadocrinus (Plate LXXIII. Fig. 3, Plate LXXIY. Fig. 4), and Steganoerinus 

 (Plate LXT. Fig. 1 e\ 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 



