164 



BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



which are connected by all intermediate stages with articulations in which the joint 

 faces are marked with irregularly meandering or more or less interrupted con- 

 centric ridges indicates that there is no fundamental difference between the 

 synarthries (figs. 1084, pi. 18; 1099, 1103, 1107, pi. 19, and 1123, 1127, 1131, 1135, 

 pi. 21) and the syzygies (figs. 1089, 1094, pi. 18) and that both are precisely of 

 the same nature as the sutures between the calyx plates in which the apposed 

 surfaces of the ossicles are marked with irregular lines, while at the same time the 



synarthry, through the 

 cryptosynarthry in which 

 the joint face sculpture is 

 obsolescent, grades imper- 

 ceptibly into the type of 

 articular union between 

 two plane surfaces as seen 

 between the radials. 



As a rule, the propor- 

 tion of nonmuscular to 

 muscular interbrachial ar- 

 ticulations in the crinoid 

 arm decreases with in- 

 creasing specialization. 



Thus in the arms of 

 IIolopus, the most spe- 

 cialized of the recent cri- 

 noids, there are no non- 

 muscular articulations. 



In the arms of the 

 comatulids and penta- 

 crinites, the next most 

 highly specialized groups, 

 the number of nonmuscu- 

 lar articulations is usually 

 between one-fourth and 

 one-sixth that of the mus- 

 cular articulations, and 

 may be even less than one- 

 tenth. Only in one or two of the smallest comatulids are the nonmuscular articu- 

 lations almost as numerous as the muscular. Furthermore, in these two groups 

 secondary arms developed to replace arms lost and the arms of multibrachiate 

 species which replace the ten arms of the young always have fewer nonmuscular 

 articulations than the primary arms which are replaced, and the arms of the 10- 

 armed young. 



In the Bourgueticrinidae the number of nonmuscular and muscular articulations 

 is approximately equal. 



218 



FIG. 218. LATERAL VIEW OF TYPE SPECIMEN OF PARAMETRA FISHERI. 



