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 



ilHBr 



IHBr 



Fia. XXIV. 



Calamocrimis diometleae. 1, posterior view of cup, showing attachment of anal tube to 

 pinnules and proximal brachials, x f ; 2, radial and proximal brachials seen from inside of 

 cup, showing attachment of interbrachials (much enlarged) ; 3, a similar portion seen from the 

 side, showing the interbrachials (unshaded) attached to the brachials and pinnulars (shaded). 

 Enlarged. (All after Al. Agassiz.) As, anus ; Ji, basal ; IBr, primibrachs ; Uir, interbnichiuls ; 

 jin, pinnulars. 



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. Uintacrinns,'Fig.ClII.)' ) 

 it is often hard to distinguish fixed 

 (From Brit. pi nnu ] ars f r0 m supplementary 

 plates. Except in primitive forms 

 (e.g. Reteocrinidae), 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 



illBr 



FIG. XXIVo. 

 Sagenocriniis expansus, to show incorpora- 



lettering see adjoining text. 

 Mus. specimen 57147.) x <j. 



