128 ECHINODERMATA—PELMATOZOA SUB-KINGDOM IIT _ 
body of the plates, enter only their outer angles. Other Fistulata have a 
madreporite. 
The ambulacra are frequently lined along their sides by perpendicular, 
wedge-shaped side-plates, and these are surrounded by movable covering pieces, both 
of which extend from the mouth to the ends of the arms. In the Palaeozoic 
Taxocrinus (Fig. 228), the covering pieces are arranged in alternate rows, with 
side pieces adjoining them. The latter plates occur also in most of the Fistulata 
and Articulata, but are rarely represented in the Camerata. The mouth may 
be exposed or closed ; being either surrounded by five oral plates (Zuzocrinus, 
Fig. 228), or the posterior oral may be pushed in between the four others, so 
as to conceal the mouth ; the latter is then said to be subtegminal (Fig. 229). 
A very remarkable modification of the ventral disk occurs in the Palaeozoic 
Camerata. Here the usually very numerous plates attain considerable thickness, 
and fit into one another like the stones of an arch to form an extremely rigid, 
more or less convex vault, which is sometimes surmounted by an equally rigid 
plated proboscis. At the apex of the dome five large-sized plates are often 

Fia. 229. 

Platycrinus Halli, Shum. Projection 
of ventral disk. a, Ambulaerals ; ia, In- 
terambulacral areas; ia’, Anal inter- 

Fic. 228. arse 3 3 ° 
radius ; e, Covering pieces of the ambu- ae ate 
“pe eee P Z : Oi aee ee : Fic. 230. 
Taxocrinus intermed- lacrals ; i, Interradials; p, Anterior 
ius, W. and 8. Ventral and lateral orals; 0, Posterior (anally Hexacrinus elongatus, Goldf. Calyx 
disk (after Wachsimuth situated) oral; 7, Plates of the anal with tegmen. «, Profile; b, Aspect 
and Springer.) . interambulacral area (after W. and 8.) from above. 
distinguishable, of which that lying in the anal interradius commonly differs 
from the rest in form and size, and appears to be wedged in amongst the others. 
These five plates are identified by Wachsmuth and Springer as orals. The 
remainder of the tegminal plates are distinguished according to their position as 
ambulacrals and interambulacrals ; in most of the Actinocrinidae the ambulacrals 
are not arranged in alternate rows (Fig. 231), but frequently consist of large 
single plates of one or more orders, which are separated from one another by 
the continuous interposition of supplementary pieces. In other groups, notably 
the Platycrinidae, the ambulacra are generally arranged in two rows of rather 
large plates, which, however, lose their original character to some extent. The 
interambulacrals usually meet with the interbrachials. The tegmen of the 
Camerata, as a rule, is composed of large nodose plates, for the identification of 
which considerable experience is required. 
Most of the Palaeozoic Crinoids have but a single opening in the tegmen, 
which is interradial in position, and undoubtedly represents the anus. With 
the exception of the Jchthyocrinidae the mouth is subtegminal, and the food 
grooves are rigidly closed. In many cases the covering pieces are pushed 
