THE FOSSIL CRINOID GENUS DOLATOCRINUS AISTD ITS ALLIES. l7 



of which (the iiiternodals) are plain disks of uniform thickness for 

 their full diameter; alternating with these at different intervals are 

 other columnals (the nodals) also thin in their median portions, 

 which are of greater diameter than the former, and have at their 

 circumference a flange which projects at either side like the tire of 

 a wheel, to a width at least equal to the thickness of one internodal. 

 Thus two nodals may abut at the exterior by their flange-like rims 

 so as to arch over the intervening internodal and completely conceal 

 it from view; or, a greater number of internodals may be interposed, 

 up to 6 or 8 or perhaps more, in wliich case the nodal stands out in 

 relief, conspicuous by its higher level and greater thickness. Fur- 

 thermore, the nodals themselves are surmounted by certain singular 

 fin-like cogs transverse to their periphery — thao is, longitudinally 

 parallel to the column. These occur to the number of three, five, 

 or sometimes more, to each nodal, of which those of successive 

 nodals may or may not coincide in position; if they do, the column 

 takes on an angular outline. The cogs may be limited to the width 

 of the nodals to which they are attached, or they may extend either 

 way for the span of several internodals, perhaps in some cases con- 

 necting and forming continuous serrated longitudinal ridges along 

 the column. Some of these cog-like projections are relatively high, 

 even equaling the diameter of the column, and they evidently in 

 some cases form a continuation of the knife-edged ridges on radials 

 and primibrachs in forms like I), spinosus, D. marshi, etc. Owing 

 to the thinness of the columnals, the peripheral flanges of the nodals 

 are held by a weak connection, so that in the fossils they may often 

 be broken off, leaving only a continuous series of columnals of the 

 same size as the internodals (pi. 9, fig. 3). 



Specimens with any portion of the column attached, or even any 

 considerable length of detached columns, are extremeh' rare, so that 

 we have little information as to the distribution of the nodal col- 

 umnals throughout the stem as a whole; but from the fragments 

 available, and from analogy with the mode of growth of the stem in 

 the crinoids generally, it is probable that in the proximal parts of 

 the stem the nodals are contiguous and conceal the interpolated 

 internodals under the arch formed by their flanges, while toward 

 the distal end the internodals increase in number, and the intervals 

 between the nodals become longer. Thus in the part of the stem 

 proximal to the calyx shown in figure 2 of plate 10 the nodal col- 

 umnals are in close contact, concealmg the internodals which maj' 

 be seen at the broken extremity, and the projecting cogs form con- 

 tinuous ridges. In figure 1 there are gaps between the nodals in 

 wliich internodals are seen, and the cogs do not always coincide in 

 alignment, both of which may be due to disturbance after death. 

 In various sections of stems figured on ])lates 9 and 10 the intervals 



