40 UINTACRINUS: ITS STRUCTURE AND RELATIONS. 



with a considerable experience among fossil Crinoids, I have never seen 

 anything to surpass it. 



The Disk. 



The disk is composed of a plated skin, the membrane being of such a 

 highly carbonaceous composition that it is jet black in the fossil. This 

 membrane evidently enclosed the entire visceral mass, and formed a lining 

 underneath the calyx plates, where it is usually seen when the plates are 

 broken away (PI. III., Fig. 1). Upon the disk it is studded or paved with 

 small calcareous plates or spicules, which are not connected by suture, but 

 are embedded in the integument without touching each other (PI. VII., 

 Fig. 2). They are of irregular shape, variable in size, and without any 

 definite plan of arrangement. The variation in size exists in the same 

 specimen, and between different specimens. In No. 71 (PI. IV., Fig. 2), and 

 also at C 3 on Plate VIII., the plates are mostly very small, but a few large 

 ones are scattered throughout the disk. In No. 75 (PI. IV., Fig. 1) they arc 

 quite large and prominent, their light color contrasting sharply with the 

 coal black perisome in which they lie. The only suggestion of arrangement 

 of these spicules is about the base of the anal tube, where they sometimes 

 take a vaguely concentric position, indicating the curvature of the tube, 

 and along the ainbulacral grooves, where there is a semblance of linear and 

 alternate arrangement. The spicules are of a porous structure, wholly dif- 

 ferent from the dense limestone which composes the calyx plates and brach- 

 ials. All the details of the disk may be seen and studied under a glass upon 

 the enlarged photographic figure made by Mr. Ricker, with infinite care and 

 pains, from the wonderful specimen, No. 75 (PI. VII., Fig. 1). The porous 

 nature of the spicules can be clearly seen in it. A greater enlargement of 

 a few spicules (PI. VII., Fig. 2) shows their irregular shape and how com- 

 pletely independent they are of any connection with each other. 



Anal Tube. 



The central part of the disk is occupied by a large, conical anal tube, 

 shaped like an inverted funnel, which is perhaps an extension of the integu- 

 ment above described. It is also composed of a plated skin, the granules of 

 the disk passing gradually into it, and becoming more and more elongate, 

 imtil toward the distal end, where the opening was, they become thread-like. 

 Yet the tube can be distinguished at once from the disk by its color. It is 



