180 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



The two cornatulid genera Marsupites and Uintacrinus illustrate opposite 

 extremes. In Marsupites (fig. 565, pi. 7) the calyx is of enormous size, with a large 

 central plate and huge infrabasals. The arms are very short and light, of exactly 

 the same structure as those of the recent comatulids. Thus in Marsupites we find 

 the most primitive calyx known of the post-palaeozoic type (hi which the interradials 

 and subradials are absent), a mass of thin subequal plates arranged in perfect 

 pentamerous symmetry and completely enclosing a globular body. The essential 

 difference between Marsupites and Uintacrimis lies in the enormously elongated arms 

 of the latter. The strain of these enormous arms upon the plates of the calyx has 

 been met by the great reduction of the calyx plates and by the incorporation of 

 numerous additional plates, brachials and pinnulars, in the body wall where they 

 perform the functions of true calyx plates. The great duplication of sutures, and 

 consequently of strong ligaments, which form a close network all over the body of 

 Uintacrinus results in the formation of a strong framework from which the long 

 arms depend, in the same manner that the car or basket of a spherical balloon is 

 suspended from the gas bag. 



The radials of the crinoids (figs. 2, p. 61, 3, p. 62, 126, p. 195, 128, p. 199, 144, 

 p. 207, and 145, p. 209) are typically the largest of the plates composing the calyx. 

 This does not indicate that they are of prime phylogenetical significance, but arises 

 from causes quite within the phylum. 



The interradial plates have become reduced from a long series in each inter- 

 radius to one hi the posterior interradius, which may be followed by a dwarfed 

 series. The infrabasals and the basals have become very greatly reduced, so 

 much so that the former commonly, and the latter occasionally, having become 

 too small for individual occurrence, unite into two pairs, leaving only one in the 

 original condition of a simple single plate. 



The reason for the progressive reduction and increasing compactness lies in 

 two developmental processes, (1) the progressive fixity of attachment, resulting 

 in a lessened power of counteracting the effect of external forces by a swaying of 

 the column, and (2) a progressive increase in the length of the arms, necessitating 

 a firmer and more compact base. Both of these factors directly affect the radials. 



Because of then- position as calyx plates they are immediately affected by any 

 force which acts upon the other calyx plates; and because of then- function of 

 bearing the arms any extension or other growth of these brings upon them an added 

 strain which they must meet. 



First of all they broaden and come into lateral contact, eliminating the inter- 

 radials and forming a closed ring very closely united with the similar closed ring of 

 basals below them. This proves sufficient for species with comparatively small, 

 short arms (see figs. 144, p. 207, and 145, p. 209); but longer arms induce a vertical 

 enlargement, giving longer apposed sides, and an inward extension, giving much 

 broader apposed sides, accompanied by an increased recumbency whereby the 

 basals, also recumbent, become attached to more or less of then- dorsal or outer 

 surface instead of to their proximal edge (see figs. 126, p. 195, and 128, p. 199). 



