SPINES. 



also the ambulacra are wide internally instead of narrow, on account of their lateral 

 The interambulacra appear to imbricate ventrally, as seen from the interior, and dip under tin- 

 adjacent ambulacra. All this is reversed when the specimen is viewed from tin- exterior 

 This complication must be constantly borne in mind in a study of internal and external 

 of fossils or confusion will ensue. 



SlMNES. 



Having discussed the corona, a brief consideration may be given to spines as an obvious 

 and salient feature of Echini. The name Echinodermata implies the character .f -pine-.. 

 It is a misnomer, if the group as a whole is considered, for in the Pelmatozoa, spines are rarely 

 developed, and in the holothurians are absent. Spines are, however, a feature of the Echini, 

 and there is no known species which is without them. In the very young plates at the <1 

 area of the test, spines are absent, not yet having developed. The nearest approach to (hi 

 condition as a species character is seen in Bothriocidaris pahkni (Plate 1, fig. 3), which it. the 

 interambulacrum apparently has no spines in the adult. It is true that minute granules 

 exist, which possibly bore spines, but such are not known. This species, how.-vrr. ha> >pim- 

 on the ambulacra and all other Echini have them on the interambulacra as well. 



Spines are classed as primary, secondary, and miliary, and are attached to tuber. 

 corresponding size. In the Palaeozoic, primary spines are especially well .level ....... 1 in Ar.-h:..-, 



cidaris, where in each interambulacral plate there is a single large central perforate tub. 



with an associated spine of variable but always relatively large si/e. In the y..unp->t plate 



at the dorsal area, as shown in Archaeocidaris rossica (Plate 11, fig. 2), neither tubercle 



primary spines are developed, and when the tubercles develop, they are at fir>t 



as seen in the younger dorsal plates in the figure cited. In Eucidaris (Plate 3 



condition exists : the young tubercle is imperforate and the young spine is a thin h 



like an inverted saucer, its spine-like character develdping in later growth. 



of a cidarid is short, broad, and distally rounded, and reminds one of the character 



of Colobocentrotus, except for the absence of marginal angles commonly . 



spines of that type by mutual pressure. 



Central primary spines are well developed in Lepidocidaris (Plate 16, 

 of larger size than associated spines and which may be fairly called primane, 

 larly distributed in Lepidocentrus (Plate 19, figs. 3-5), in Hyn^dnnu, rwuf* 

 fig 6) and on the adambulacral columns in Pholidocidaris (Plate 7. fig. : 

 the tubercles, in part in Perischocidaris (Plate 67, fig. 2). In the ,,,n ; , 

 there are also small secondary spines clustered around the primaries and 

 ambulacral plates (Archaeocidaris). Or secondary spines and tubercles may , 



