20 UINTACRmUS: ITS STRUCTURE AND RELATIONS. 



groove (PI. VIIL, Figs. B7 and C 3), but shows no indication of any ambu- 

 lacral skeleton. Neither covering plates nor side pieces are discoverable. 

 I have specimens in which the preservation of these parts is exquisite ; the 

 ventral grooves are almost as sharp and distinct as in an alcoholic specimen. 

 Considering the extremely firm and fine-grained chalky matrix, which has 

 preserved even the membrane of the disk, and which I have with refer- 

 ence to this point cleared away under a strong magnifier with the finest 

 instruments, I feel certain that if covering plates existed they would have 

 been preserved, and would be visible in some of the specimens, no matter 

 how delicate they were. From the fact that with such material as this the 

 most careful search fails to disclose any trace of such plates, my experience 

 in the preparation of many Palaeozoic Crinoids leads me to feel entirely 

 confident that there were none. In this respect Uintacrinus differs from 

 the Pentacrinidae and the Inadunata generally, so far as we know the 

 structure of these parts. 



As to the pinnules, their arrangement, structure, and shape, and the 

 appearance of the ventral side of both pinnules and arms, no description 

 could give half so good an idea as may be derived from a careful study of 

 the photograph on Plate VIII. under a good magnifier. It is taken from a 

 detail of slab No. 1, which is one of the finest examples of fossil preserva- 

 tion ever found. It was cleaned with great labor and patience by one of 

 the most skilful preparators. The photograph is about .58 natural size. 

 I am much indebted to Mr. Ricker and Mr. F. M. Fultz, of Burlington, 

 Iowa, for their intelhgent assistance in making this photograph. It is rare 

 indeed to find in a single specimen such an amount and variety of informa- 

 tion as is contained in this one. Almost every one of the important points 

 discussed in this paper will find a good illustration on this plate. 



In the proximal portion of the pinnules the pinnulars are much wider 

 than long. They are wedge-shaped, widest on the dorsal side or back, and 

 sloped toward the ventral side so that the inner edge is the thinnest — thus 

 making the pinnule, in a side view, appear deeply serrated toward the 

 ventral side. This facilitates mobility, enabling the pinnule to coil upon 

 itself to some extent. But besides this there is a sloping outward of the 

 lower side of each ossicle, so that seen from the back the pinnule appears 

 deeply notched, or serrated on each lateral margin. This structure is well 

 shown on Plate V., Fig. 1 ; and especially the photographic figure 5 on 

 Plate VII. This sloping imparts a lateral bending or twisting motion to 



