BASAL PLATES IN CRINOIDEA CAMERATA 539 



area by growth pressure would be expected, but it does not occur. 

 On the contrary, the posterior radials either assume an asym- 

 metrical outlfhe as in Antedon, most of the hexagonal Camerata 

 (Pis. II-III), and many of the Fistulata, or the continued widening 

 must be compensated for by accelerated growth of the sarcode in 

 the posterior region. Since no better opportunity for simple plate 

 migration could be conceived, and since it does not in this case 

 occur, there is little reason for believing in its existence. 



Complex migrations consist of plate shif tings induced by accel- 

 erated growth, tortion of the body wall, in either local or broad 

 areas, which distorts the normal space and contact relation of 

 plates. This form of migration is diagrammatically shown in the 

 carrying up by elongation of the anal tube of the anal in Antedon, 

 and of the radianal in Promacocrinus and Hathrometra. Where 

 the anal and radianal, being more firmly attached to the viscera 

 than to the adjacent plates, are bodily lifted out of the cup into 

 the tegmen by the accelerated growth of the hind-gut, this process 

 is the one which undoubtedly explains the migration of the radianal 

 in all fossil crinoids. A more common form of complex migration, 

 and one shown in many groups of crinoids, is that which results 

 in the formation of biserial from uniserial ossicles in the arms. 



e) Plate interpolation. — This process may be defined as the 

 interpolation of some plate or plates, of primary or secondary 

 derivation, between any plates forming the primitive crinoid cup 

 and its appendages. It is one of the most common forms of evolu- 

 tion found in the crinoidea, and may be broadly separated into 

 two groups: primary interpolation, or the development of primary 

 or secondary plates in situ from a primary or secondary formative 

 cell group; and secondary interpolation or migration. Only the 

 first type need here be considered, as plate migration has already 

 been discussed. Primary interpolation is the only known method 

 by which additional stem ossicles appear; the new ossicles devel- 

 oping either between the base and the adjacent stem ossicles, as 

 in the Inadunata and Camerata, or between other stem ossicles, 

 as in the Flexibilia. In the development of cirri, primary inter- 

 polation is the rule, the interpolation taking place at the proximal 

 end of the appendage. Here too belong the development of the 



