390 SMITHSONIAN INSTITUTION 



near the calyx of very thin columnals and as far as known tapering rapidly 

 downward. 



This species is notable for being the one in which the true structure of the tegmen in 

 the Flexibilia was discovered. An elaborate description of it was given by Wachsmuth and 

 Springer in 1888, and their figure of the principal specimen has been repeatedly copied in 

 general works on paleontology. I give a somewhat better figure of the ventral side, enlarged 

 to two diameters (PL LIII, fig. lb). The tegmen consists of a thin and pliant skin, thickly 

 paved with small calcareous plates in close contact, but without definite arrangement. At the 

 oral center there is an open mouth surrounded by five large unsymmetrical parted oral plates, 

 elliptic in outline and of three sizes. The posterior oral is very much the largest, the two 

 anterior less than half as large, and the two lateral orals again scarcely half as large as the 

 latter ; these plates are all quite thick, and the smaller ones, from which the perisome has 

 somewhat fallen away, stand out like pegs. Five large, strongly plated ambulacra, with 

 short, interlocking covering plates, pass out from the mouth between the orals, bifurcating 

 over the radial axillaries, and traversing the perisome to about the lower IIIBr — thus giving 

 two bifurcations before the arms become free. The anal series of plates is attached to the 

 perisome at the posterior side and finally merges in it ; the folds in the perisome where the 

 tube is pushed to one side can be well seen in figure la. 



The specimen in which these structures are so beautifully preserved was obtained by 

 Wachsmuth and myself in the summer of 1888, when upon a collecting expedition at the 

 extensive quarries in the Kinderhook limestone at Legrand, Marshall County, Iowa. It has 

 been slightly compressed vertically, and as discovered the rays were broken off ; the upper 

 part of the specimen was imbedded in a very fine-grained, perfectly homogeneous deposit of 

 firm, calcareous mud. The favorable position of the expanded rays, coupled with the 

 unusually fine preservation of the crinoids of that locality, encouraged the hope that we 

 might have here the means of ascertaining the actual ventral structure of this group, as to 

 which there had been much and diverse speculation. This hope was realized after long and 

 patient cleaning of the specimen by removing the matrix with fine tools under a strong magni- 

 fier ; and the result was a complete modification of previously prevailing views as to the 

 essential tegminal structure of the Flexibilia and the classification of the group, as was set 

 forth in the publication which followed. It is rare in the history of paleontology that the 

 discovery of a single specimen has thrown such a flood of light upon the larger questions 

 affecting a given group. The character and extent of the perisome, without the ventral 

 structures, are well shown in the three other large specimens figured on Plate LIII. The 

 sutures in several of the specimens do not appear to be very sinuous in the lower parts ; this 

 is largely due to the effect of cleaning from a very fine adherent matrix, and to the unusual 

 thinness of the patelloid projections in this species ; but the usual structure prevailed to a 

 moderate degree. 



This species marks the opening of a line of development in Taxocrinus, characterized 

 by a profusion of strong interbrachial plates forming an essential part of the calyx wall, 

 which continued through a number of well-marked and abundant species until the extinction 

 of the genus. The last three small species surviving in the Warsaw to Kaskaskia are very 

 much like it in habitus. It exhibits some variability in the construction of the calyx as to 

 the generic character based upon the number of primibrachs, some specimens having in part 

 of their rays only 2 IBr instead of 3. This instability looks more formidable in the figures 

 than it really is, because the specimen showing it most conspicuously happens to be very 

 thoroughly figured (PI. LIII, figs. 2a, b, c). In six specimens in which 24 rays are exposed, 

 18 of them have 3 IBr, and 6 have 2, and in the latter there is a tendency to compensate for 

 the deficiency in IBr by an increase of IIBr ; out of 42 rami showing the IIBr, 34 have 3, 

 and 8 have 4. Hence there can be no doubt that the normal structure in the species is 3 IBr 

 and 3 IIBr. 



