652 THE SPINES. 



and breadth of the smaller wedges intercalated between the older ones, as the 

 spine increases in the periphery, the outer edge of the homogeneous centre 

 is increased by the addition of smaller cells like those of the central part 

 of the shaft. The shape and mode of connection of the wedges of the outer 

 sheath are frequently quite characteristic of the different genera and species 

 (PL XXXV. f. 1-5). The structure of the papillae is more homogeneous; 

 they have no regular outer sheath of wedges, as in the primary interambu- 

 lacral spines. They resemble closely in their structure the spines of Arba- 

 ciadae. 



As the spines of Cidaridae are among the most common fossil remains in 

 some of the formations, I have given detailed figures of the spines of several 

 of the genera, to show the variations which may occur in the same species, 

 either in size or in shape, according to their position on the test. These changes 

 are limited to the interambulacral spines. The ambulacra] papillae, though 

 differing so strikingly from the interambulacral primary spines, yet are not 

 variable, and show no differences of consequence in the course of the ambu- 

 lacra] areas. The papillae which surround the base of the primary interam- 

 bulacral spines are similar in structure to the ambulacra] papillae, there being 

 no other primary spines in the ambulacra] system of the Cidaridae. 1 need 

 not go beyond referring to the explanation of the plates (PI. I c ., I'.), to 

 show how impossible it is to base specific distinctions upon the structure of 

 the spines, even when they come from the same part of the test. Compare, 

 for instance, the figures of the abactinal spines of Phyllacanthus gigantea 

 (PI. I'-f. 27-29). The great differences observed in the spines of Phylla- 

 canthus baculosa are specially noteworthy (PI. I' f sj, -.is; PL I*, f 

 11 -Jo"). In all Cidaridae we find certain primary spines frequently quite 

 smooth, slender, pointed, in the midst of the other spines ( PI I'.f. 17, 88,' 

 PL. I'.f. 22,85). Compare especially the different spines of Phyllacanthus 

 dubia (PI. I'.f. 7-10) and of Phyllacanthus imperialis (PL I f .f. ,:). In sev- 

 eral of the genera the tips of the primary spines are often cup-shaped, and 

 this, added to the other structural differences, often produces a very marked 

 contrast in specimens of the same species in which one or the other type of 

 spine may predominate. Compare the spines of Goniocidaris tubaria ( PI 

 I'.f 9-u; PL I'.f. 32-36), and of Goniocidaris geranioides (P£ I'f.15- 17). 



The spines of the actinal surface are usually flattened, elongate, in Gonio- 

 cidaris canaliculata. and in Phyllacanthus (PL I'.. PL l'.)\ in Porocidaris 

 they assume a very peculiar shape (PL I'.f. ',"", ;/ i ; in Dorocidaris they are 



