THE CRINOIDEA 



containing minute supplementary plates (Isocrinus, Calamocrinus) ; 

 thus the thecal cavity is enclosed by secondary as well as primary 

 elements. Similar plated membrane may occur between the IIBr 



IB? 



illlBr 



IIIBr 



or IIIBr of a single arm. The small plates may increase in size and 

 firmly bind together the arms and rami (Fig. XXIVa) ; those 

 between brachia are "interbrachials" (iBr); between IIBr, "inter- 



secundibrachs " (illBr), and so on. 

 Similarly there are interambula- 

 crals of various orders ; the ilAmb, 

 merging at the sides of the theca 

 into the iBr; the illAmb separated 

 by the thecal cavity from illBr ; 

 the illlAmb, and so on. The 

 interbrachials sometimes, though 

 rarely, descend between the 

 radials. Not merely brachials, 

 but also pinnulars may be incor- 

 porated in the cup, and between 

 pinnules are developed " interpin- 

 nulars" (e.g. Uintacrinus, Fig. CIIL); 

 a plT<tl Ta?te b r^TLrd f ed) C ;t it is of ten hard to distinguish fixed 



lettering see adjoining text. (From Brit. p i nnu l ars f rom supplementary 

 Mus. specimen 5il4i.) x jj. . . r V . . * 



plates. Except in primitive forms 



(e.g. Eeteocrinidae), the interbrachials have a similar arrangement 

 in each interradius of an individual. Indeed^ the arrangement often 

 serves as a means of distinguishing species and genera. This 



UlBr 



FIG. XXIVn. 

 Sagenocrinus expansus, to show incorpora- 



