26 SMITHSONIAN INSTITUTION 



helping to increase the capacity of the visceral cavity ; that the latter appear in 

 the growing crinoid, but are unrepresented in some of the groups ; that they are 

 subject to great modification, and therefore have a high taxonomic value. The 

 primary plates were subdivided into two classes; plates of the abactinal, and of 

 the actinal systems; the former including all plates connected with the cham- 

 bered organ and axial cords, the latter those connected with the mouth and 

 food canals; the former being represented by the columnals, infrabasals, basals, 

 radials, and plates forming the dorsal parts of arms and pinnules, the latter by 

 the orals and ambulacral plates of tegmen and arms. All the others were 

 shown to be supplementary pieces, being the interradial or interbrachial, and 

 the interambulacral systems. A general morphological discussion of these ele- 

 ments followed, applicable to the crinoids of all groups, which has since been 

 supplemented by the great work of Bather in the Lankester Zoology; and by 

 these the whole subject has been thoroughly covered, so that there is no need 

 for me to enter upon it here. But I shall follow substantially the same order 

 as in the Camerata in describing the skeleton of the Flexibilia. 



The dominant general character of the group Flexibilia is the distinctive 

 facies due to its strong, well-united, and flexible calyx passing almost insensi- 

 bly into the arms. As Bather has well expressed it, 1 " it is the combination of 

 massiveness with flexibility that characterizes the Grade." These features are 

 produced, the first by the thickness of the plates, and the second by their mode 

 of union with each other. This union is by muscular articulation between 

 plates of the radial series, while between these and the supplementary plates, 

 and the latter among themselves, it is of the type of loose suture ; both are char- 

 acterized by fossae for the lodgment of ligament bundles, and contact areas of 

 crenulated surfaces. Upon the relative predominance of these elements de- 

 pends the greater or less looseness of the union and consequent flexibility of 

 the calyx. In this there is much variation, from that of forms like Forbesi- 

 ocrinus nobilis with extremely large and deep ligament fossae and narrow bands 

 of contact surface (PI. XXIV), to that of F. saffordi with shallow and re- 

 stricted ligament fossae and numerous small contact areas (PI. XXXI). In 

 some forms with globose calyx and strongly differentiated arms, like Nipter- 

 ocrinus (PI. XV), it is probable that the union among the calyx plates was 

 nearly that of close suture; these are in some respects transition forms toward 

 the Inadunata. Added to the foregoing general characters is that of the arcu- 

 ate or wavy sutures between plates of the brachial series, to be described pres- 

 ently, which imparts a highly characteristic appearance to the specimens. 



In size of the specimens there is considerable diversity, the genera of the 

 globose type such as Lecanocriniis being in general rather small, some not 



1 Lankester's Zoology, pt. 3, 1900, p. 187. 



