50 SMITHSONIAN INSTITUTION 



Many years after our discovery of the tegmen of Taxocrinus, I succeeded 

 in exposing that of Onychocrinus in several specimens in a manner which 

 threw additional light upon the structure. It was described and illustrated in 

 1906, 1 and to the description there given, as well as herein under Onychocrinus 

 ulrichi, must reference be had for the full details. The immense size of the 

 posterior oral is emphasized, and the very strong ambulacrals (much heavier 

 than those of Taxocrinus) are shown, together with a conspicuous anal tube 

 located close behind the oral. The perisome is extensive, very thin and pliable, 

 thickly studded with very small plates passing far down between the rays, and 

 meeting strong interbrachial plates with which the perisome connects by a well- 

 defined margin of attachment (Pis. LXVI, LXVII, LXVIII). This structure 

 is in every way analogous to that of Ptilocrinus (PL LXXV, figs, la, b, c), 

 in which the perisome also passes down between the rays, and in the lower part 

 is studded with strong interbrachial plates, not so regularly arranged as those 

 of Onychocrinus. The tegmen of Calamocrinus, which has a similar strong 

 interbrachial perisome, represents a stage more advanced than this, in which 

 the orals have been almost completely resorbed, it being difficult to identify 

 them except in a sectional lateral view, where they look like long pegs. 2 



The tegmen of such Flexibilia as Onychocrinus, with its wide-spreading 

 rays and flexible calyx, must have been extraordinarily pliant, perhaps more so 

 than in the living crinoids. As found in the fossil, it is always pressed down 

 to the bottom of the calyx cavity so that the connection of the several com- 

 ponent parts is not so readily seen in the drawings. Their proper position will 

 be better understood from the restoration (PI. LXVIII, fig. 6) constructed 

 after careful study of the figured specimens and some others, showing the 

 tegmen as it must have been in life, especially the relation of the anal tube to 

 the perisome, which is always disturbed in the fossil; and also from text-figure 8 

 showing the relation of the tube to the adjacent calyx plates, and the vertical 

 groove in the margin of the radial to the left caused by pressure of the tube. 

 No- description of this tegmen and its appendages could be as luminous and 

 instructive as these restorations, taken in connection with accurate figures of 

 the truly extraordinary series of laboriously prepared specimens illustrated 

 upon the above-cited plates. 



The ambulacrals in the few specimens in which they have been seen vary 

 somewhat in form and proportion; in Taxocrinus they are short and wide 

 (PI. LIII, fig. lb), somewhat like those of Ptilocrinus, while in Onychocrinus 

 they are elongated to a degree not seen in any other crinoid (PI. LXVII). 



The interambulacrals in the Flexibilia consist wholly of small plates form- 

 ing with the perisome an integument, or plated skin, which supports the 



1 Journal of Geology, vol. 14, 1906, pp. 467 et seq. 



'' Agassiz, A., Mem. Mus. Comp. Zoology, Harvard, Vol. XVII, 1892, pi. 6, fig. 7. 



