DISCOID CRINOIDAL ROOTS AND CAMAROCRINUS 245 



the top consists of calcite plates, increasingly numerous in larger 

 specimens (Figs. 12 and 13). The plates are hexagonal to quad- 

 rangular, and arranged radially in short intercalating rows. The 

 inner part of the plates appears in thin-section less dense than the 

 outer, and is rough surfaced. Radiating ridges are also evident 

 (Fig. 14), on the inside. Maceration may have reduced them, but 

 they are not now seen to touch the fixing plate. 



The fixing plate is thin and closely knit to the surface of support. 

 It is a single large plate, united by suture only around the periphery 

 of the root. On the upper surface of the fixing plate there are raised 

 lines or ridges. These are well seen in one specimen from which the 

 top plates are partly broken off, and in several others which have 

 been weathered. None are complete, and a diagram (Fig. 15), is 

 given, for the sake of completeness and clearness also, instead of a 

 drawing of a single specimen. There is a central sharp elevation, 

 from which radiate five angular ridges, and these are widest and 

 highest at a short distance from the center. Other ridges intercalate, 

 chiefly in diverging pairs, and all diminish in intensity toward the 

 periphery. 



The internal structure might be quite geometrically symmetrical 

 if the external form were so, but in every specimen the place of 

 growth has modified the form of the root more or less. The fixing- 

 plate follows closely the surface of support, whose irregularities are 

 reflected within. Again, some specimens have the stem-scar near 

 or at one side, and directed obHquely to that side as if the surface of 

 support had been vertical or incHned, the stem too having been 

 vertical. In those cases the longer side is in every way the more 

 developed. Figure 13 shows a root which had grown over the edge 

 of the supporting object. 



Figs. 16 and 17 represent a specimen which has grown on a slender 

 branch of Pachydictya acuta Hall and surrounded it, the fixing-plate 

 thence having bedded to an irregular, probably muddy, surface. The 

 marginal lobes are extended as round short roots. Two of these, 

 which are cleaved off, show central circular canals. One other 

 specimen which was found near this one has grown in a similar way 

 but has flat areas around the top. 



This type of discoid base I find in greater numbers and through 



