BASAL PLATES IN CRINOIDEA CAMERATA 



497 



edges are thus denned, the plates steadily increase in size, appar- 

 ently by interstitial growth. 1 The adjacent borders of the plates, 

 however, do not come into absolute contact, as a thin lamina of 

 sarcode is interposed between them until the sutures are closed by 

 anchylosis. The upper margins of the basals have at this time no 

 distinct border, 2 but are still growing by the process of branching 

 and anastomosis (see p. 501, plate growth). 



3 6 



Fig. 2. — Formation of the dorsal cup and migration of the anal plate in Antedon 

 rosaceus: 1, 2, after Thomson; 3-5, after Carpenter; 6, original from specimen in 

 Oberlin College Museum; = orals; b = basals; r = radials; a = anal. 



Shortly after the fixing of the pentacrinoid and the opening of 

 the cup, a third series of plates, the radials, make their appearance 

 in the space left by the beveling off (absorption) of the adjacent 

 lateral angles of the basals and orals (Fig. 2, No. 2), 3 the beveling 

 being caused apparently by the encroachment of the radials. 4 About 

 the period of the development of the second radials (costals) a 

 forked spicule makes its appearance between the upper parts of 

 the posterior radials. This plate gradually increases in the regular 



1 Interstitial growth: ref. 35, p. 538. 3 Ref. 35, p. 539. 



2 Ref. 11, p. 729, PI. 41, Fig. 1, b. 4 Ref. 11, p. 729. 



