74' 



CIDARIS 



Fig. 1. 



"From the above Table it will be seen that this variation in form is independent of the 

 size of the test, and is dependent rather npon some peculiar law in the formation of the 

 spine, or some cause which has contributed to produce a greater development of calcareous 

 matter in one part than in another. In flints which contain the spines of C. cla- 

 vigera a fracture passing through the spine will often exhibit this growth very beauti- 

 fully ; thus, in the woodcuts (fig. 1), whilst an earlier form of 

 the spine is clearly defined, the subsequent addition of 

 material is also manifested by the change of tint. The same 

 effect can also be observed in longitudinal sections of the 

 ordinary spines, a difference of density and of hardness in 

 the whole or parts of the enveloping layers being very 

 apparent. 



Sections of body-spines of C.clavigera 

 in flint. 



" In C. davigera the difference between the spines of the peristome and of the ambitus is 

 more marked than in most of the other species of the Cidarid^ of the Upper Cretaceous 

 group. The woodcuts (fig. 2) drawn from the spines of the tubercles adjacent to the 

 mouth, and magnified four diameters, show that their apex is more acute, their ridges fewer, 

 and their body more elongate-ovate than in the larger spines of the ambitus. The spines 

 of the granules (fig. 3) are also dissimilar, being longitudinally striated, contracted at 

 intervals, having almost parallel sides, and being in transverse section ovate. 



Fig. 2. 



Fig. 3. 



Spines of C. clavigera from the tubercles adjacent to the peristome ; 

 magnified four diameters. One spine has four serrated ridges, 

 the other six. 



Spine of C. clavigera from the granules on the 

 margin of the ambulacral areas ; magnified 

 eight diameters. 



" The spines in their original condition appear to have been tinted with parallel bands 

 of colour, perpendicular to the axis. In several examples now in my cabinet the apex of 

 the spine shows evidence of this peculiarity ; but in one specimen in particular (a test to 

 which the spines are attached) that circumstance is so marked and is so persistent (the 

 base and apex of the body of the spines being specially affected) that it can hardly be the 

 result of accident. 



" The common longitudinal perforations in the outer layer of the spine alluded to at 

 p. 50 seem to have been chiefly due to disease or to some difference in structm'e which 

 caused those parts, now empty, to decay with greater facility in one direction than in 



