180 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



The two coinatulid genera Marsupites and Vintacrinus illustrate ojjposite 

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

 central ]ilate and luige infrabasals. The arms are verj^ short and light, of exactly 

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

 the most primitive calyx known of the post-palieozoic type (in which the intcrradials 

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

 pentamerous symmetry and completely enclosing a globular body. The essential 

 difference between Marsupifts and Vintacrinus lies in the enormously elongated arms 

 of the latter. The straui 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 ])lates, brachials and pinmdars, 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 

 Vintacrinus 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 thej^ 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 in 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 fLxity of attachment, resulting 

 in a lessened j)Ower of counteracting the effect of external forces bj' 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 their i)osition as caly.x plates they are immediately affected by any 

 force which acts upon the other caly.x plates; and because of their 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 closch' united with the similar closed ring of 

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

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

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

 broader apposed sides, accompanied by an increased recumbency wherelw the 

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

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



