48 SMITHSONIAN INSTITUTION 



just to the left of the anal tube, and in other six-rayed specimens; and where 

 the process is carried to its ultimate conclusion the ten-rayed Promachocrinus 

 (sensu P. H. Carpenter) is produced. 



II. The Actinal System 

 a. The Tegmen 



The tegmen includes the interambulacral plates or integument of the 

 perisome, which belong to the category of supplementary plates, and logically 

 should be considered under that head; but it seems more convenient to treat 

 the tegmen here as a whole. It consists of orals, ambulacrals, and inter- 

 ambulacrals, out of which open the mouth and anus. 



The tegmen of the Flexibilia for a long time was totally unknown and is 

 still very rarely seen; its construction was the subject of much theoretical dis- 

 cussion in former years, which may now be relegated to ancient history. 1 Its 

 true nature was first discovered and described by Wachsmuth and Springer 

 in 1888, 2 in a specimen of Taxocrinus intermedins of remarkably perfect preser- 

 vation, which shows the essential structure completely (PI. LIII, fig. lb). The 

 great and illuminating feature was the uncovered mouth opening through a 

 pliant disk as in the Recent crinoids, thus differentiating it from that of all 

 other known Paleozoic crinoids, and fixing the systematic position of the group 

 in a way that put an end to the chief speculative discussion concerning it. For 

 it was inferred with much confidence that the structure found in this genus 

 was that of the group throughout— an inference which subsequent discoveries 

 have amply confirmed. A similar tegmen has since been found by me in 

 another species of Taxocrinus, and in the genera Homalocrinus, Cholocrinus, 

 Pycnosaccus, Synerocrinus, and Onychocrinus, representing three out of the 

 four families of the Flexibilia (Pis. VI, fig. 15b; IX, fig. zpi; XI, fig. 15b; 

 XLII, figs. 8a, 11; LXVII, fig. 7). 



The mouth is surrounded by five parted orals, between and below which 

 the plated ambulacra pass inward after traversing the perisome from the arm 

 bases. The orals occupy a relatively small space at the center of the disk, and 

 represent a late stage in the ontogeny of the young crinoid such as was de- 

 scribed by W. B. Carpenter in his memoir on Anted on rosacens, 3 where the 

 circlet of orals has detached itself from the summit of the radials on the 

 shoulders of which it was previously superimposed, and is progressively car- 

 ried inward by reason of the outward expansion of the ring of radials — the 

 space between the two series being now filled in only by the membranous 



1 See N. A. Crinoidea Camerata, pp. 88-124. 



" Discovery of the Ventral Structure of Taxocrinus, Proc. Acad. Nat. Sci., Philadelphia, vol. 40, 



PP- 337-363. 



3 Philosophical Transactions, London, 1866, p. 727. 



