TAXOeiUJNFIDAE 429 



they may vary in size or shape in different specimens. In figure 7 the! posterior oral is 

 transversely bisected, which is not observed in any other case. That this plate is a madre- 

 porite rather profusely perforated is best shown by figures ga, b, which are enlarged views 

 of the central part of a small tegmen in which the pores are quite plain. 



The greater portion of the disk was clearly membranous, with calcareous spicules or 

 granules imbedded in it ; except at the oral center and the ambulacral regions, which are 

 occupied by rows of large, elongate, alternating plates more or less tumid exteriorly, and 

 keeled on the under surface as if for lines of muscular attachment. The ambulacra extend 

 from the oral center along the rays to the point of bifurcation, beyond which they have not 

 been traced ; but they no doubt follow the arms in a modified form. The three anterior 

 ambulacra pass in between the bases of the- four small orals, much as they do between the 

 plates of the oral pyramid in Holopus (PI. A, fig. 8). The two posterior ambulacra first 

 meet the outer corners of the large oral, thence run along its edges and pass in toward the 

 mouth at the junction between it and the smaller orals. 



Whether these strong alternating plates were covering pieces arching over the food 

 grooves, or were merely some kind of sub-ambulacral plates forming a support for more 

 delicate plates which were not preserved, or for an ambulacral structure composed only of 

 soft parts, I am unable to determine. The variation in size and shape of these structures in 

 the living crinoids is very great, and there is no reason why plates as large as these may not 

 have served as covering pieces, opening out when the orals did to admit the inward passage 

 of the food-bearing currents. The membranous connecting integument between the ambu- 

 lacra was reinforced by a close growth of calcareous .particles, similar to what has been 

 observed in Taxocrinus, Uintacrinus, and the disks of many Recent crinoids (fig. ga). It 

 extends far out along the rays, bordering the ambulacra as far as they have been traced. 

 It is extremely frail in the fossil, and is of a slightly darker color than the calyx plates. 



Figure 8 throws further interesting light upon these structures, being the under or 

 opposite side from that which we have seen in the others. The preparation was made by 

 removing the calyx plates from the dorsal side — as if the specimen in figure 1 were stripped 

 of its plates. We see the under side of the posterior oral, somewhat rough or wrinkled, and 

 of some of the others at the angles between the ambulacra ; also the large opening, or mouth, 

 which is roofed over by the pyramid of small orals in figure 7; and the keeled under side 

 of the ambulacra is quite plain. 



Figures 11a, b are from a dissected calyx of 0. diversus showing the form of the pos- 

 terior basal, with its rounded upper margin, and exterior socket supporting the anal tube- 

 plates ; it is further interesting here as showing the much smaller size of the tube in this 

 species than in 0. ulrichi, where the socket occupies about the entire distal width of the 

 plate (figs. 1, 4). 



On Plate LXVIII, figures 1, 2 and 3 give further views of the same tegminal structures ; 

 and figure 1 an especially good picture of the distal aspect of the clusters of folded arms, 

 as they are also seen in figure 4. The latter, in which no part of the tegmen can be identified 

 except the foreshortened posterior oral, is important for the unusual way in which it exposes 

 the interbrachial system. Although extended almost horizontally, the margins of the rays, 

 and of the interbrachial areas between them, are bent inward in a peculiar manner (as if 

 especially intended to illustrate Meek and Worthen's description of the Onychocrinus plan), 

 so as to bring into view the interbrachials around the entire margin from the first ramule 

 of one ray to that of another ; in some of these interradii there are 25 plates. 



Figure 5 is the dorsal view of a very symmetrical and horizontally extended specimen 

 showing the relative proportions of calyx and rays when in that position, for comparison 

 with similar specimens of O. diversus. Figure 6 is an attempt at restoration of the tegmen 

 from the evidence of all the foregoing preparations, with special reference to the position 

 and functions of the anal series of plates as a backing or support of the rectum, which is a 



