SPINES. 77 



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

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

 adjacent ambulacra. AH this is reversed when the specimen is viewed from the exterior. 

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

 of fossils or confusion will ensue. 



Spines. 



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 of spines. 

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

 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 dorsal 

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

 condition as a species character is seen in Bothriocidaris pahleni (Plate 1, fig. 3), which in 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, however, has spines 

 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 tubercles of 

 corresponding size. In the Palaeozoic, primary spines are especially well developed in Archaeo- 

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

 with an associated spine of variable but always relatively large size. In the youngest plates 

 at the dorsal area, as shown in Archaeocidaris rossica (Plate 11, fig. 2), neither tubercles nor 

 primary spines are developed, and when the tubercles develop, they are at first imperforate 

 as seen in the younger dorsal plates in the figure cited. In Eucidaris (Plate 3, figs. 1, 2) a similar 

 condition exists: the young tubercle is imperforate and the young spine is a thin hollowed plate, 

 like an inverted saucer, its spine-like character developing in later growth. A young spine 

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

 of Colobocentrotus, except for the absence of marginal angles commonly developed in the 

 spines of that type by mutual pressure. 



Central primary spines are well developed in Lepidocidaris (Plate 16, figs. 1-3). Spines 

 of larger size than associated spines and which may be fairly called primaries, are found irregu- 

 larly distributed in Lepidocentrus (Plate 19, figs. 3-5), in HyaUechinus rarispinus (Plate 23, 

 fig. 6), and on the adambulacral columns in Pholidocidaris (Plate 73, fig. 3), and, judging from 

 the tubercles, in part in Perischocidaris (Plate 67, fig. 2). In the several types considered, 

 there are also small secondary spines clustered around the primaries and alone existent on the 

 ambulacral plates (Archaeocidaris). Or secondary spines and tubercles may occur with pri- 



