248 PALAEONTOLOGY 



radials are five in number, but when we have identified 

 the basals and these we still have a great many plates 

 in the dorsal cup unaccounted for. Nevertheless, we 

 cannot have mistaken the radials, because there are 

 plates directly above them which lead up to the arms, 

 and that could never be the case with basals. These 

 plates above correspond with the lower brachials of 

 Citpressocrinus, but instead of forming part of free arms 

 they are incorporated in the dorsal cup : they are called 

 fixed brachials. But holding them fixed are other plates, 

 interradial in position', arranged in bifurcating series : 

 these are called interbrachials. The largest of them dis- 

 turbs the five-rayed symmetry by pushing itself between 

 the radials and meeting two of the basals : it is called 

 the anal, not because it carries the anus, but because it 

 forms the base of the interray, high up in which lies the 

 anus. 



The ventral surface, instead of being flat and of few 

 plates, as in Cupressocrinus, is a lofty dome of many 

 plates, solidly articulated together. Crinoids with this 

 feature are confined to the Palaeozoic, and are known as 

 Camerata. This arrangement closes in the mouth com- 

 pletely and shuts it off from all possibility of the excre- 

 ment from the anus fouling the food. As an additional 

 precaution, the anus is lifted up from the level of the 

 tegmen by a short anal tube. 



The arms branch into two immediately they become 

 free from the dorsal cup, and again branch repeatedly. 



3. Marsupites testudinarius (Fig. 69) is a crinoid 

 characteristic of a definite zone high up in the White 



