492 



NORTH AMERICAN INDEX FOSSILS, 



food grooves and generally comprise two alternating rows of small, 

 somewhat regularly arranged plates which are always movable on 



the arms and pinnules but on the 

 tegmen only in those crinoids in 

 which the mouth is exposed. In 

 some of the Camerata these 

 plates are so large as to be dis- 

 tinguished as radial dome plates. 

 In the FisTULATA the posterior 

 side of the ventral disk is pro- 

 longed upward into a large tube, 

 the ventral sac or ventral tube; 

 this may or may not contain the 

 anus, but if it does, the opening 

 is usually on the anterior side of 

 the tube. The anus may be cen- 

 tral in the ventral disk or any 

 where between its center and the 

 margin. Its position determines 

 the posterior side of the calyx. 

 It may open directly through the 

 tegmen, from the anterior side 

 of the ventral tube as in the 

 FiSTULATA, or at the end of 

 a special tube, the anal tube. 



The mouth may be tegminal, 

 opening through the ventral 

 disk, and surrounded by the 



Fig. 1806, Diagrams of crinoid arms. 

 a, Platycrinus hemispherifus, showing the 

 change in the form of the plates from uni- 

 serial to biserial and hack to uniserial in 



the apical portion of the arm ; b, c, P. ends of the ambulacra and by 



/m«^.z/;7/.r; ^ a young individual in which ^j^^ ^^.^j^ ^y^^^ present, Or by 

 all plates are uniserial ; r, part of adult 



showing change to wedge-shaped in 6th the mterambulacral plates; or 



plate and biserial condition after the 7th suhtegminol, when completely 



plate; d, Dichocrinus inornatus,2id\x\\.Birm t-QQfpr| over bv the orals or 



showing change to biserial after the 5th . 11, 1 /t^- 



and back to uniserial after the 84th plate, mterambulacral plates ( Fig. 



(After Grabau, Am. Journ. Sci., XVI., 1805, rt). From the mOUth 



pp. 289-300, 1903. ) radiate the ambulacra to the tips 



of the rays following the ventral furrows of the arms and pin- 

 nules. When subtegminal they enter the calyx through the arm 

 openings at the upper edge of the dorsal cup (Fig. 1805, 0) ; when 



