376 



ECHINOZOA. 



the genus occurs in the Silurian rocks, but a number of forms are found 

 in the Carboniferous Limestone. 



In the genera Melonites and Oligoftorns, of the Carboniferous rocks, 

 we have large spherical Urchins, in which the test appears to have been 

 rigid, though some of the plates are occasionally bevelled off, so as to 

 articulate in an overlapping manner with one another. In Melonites 

 (fig. 247) there is a multiplication of the plates of both the interambulacral 

 and ambulacral areas, the former consisting in the middle of seven or eight 

 rows, while the latter are of eight or ten rows, or, in a British species, of 

 from twelve to fourteen rows. The central two rows of ambulacral plates 

 are larger than the rest and elevated above them, and each plate of these 

 areas is doubly perforated. The apical disc (fig. 245, b) is composed of 



Fig. 247. — A, Portion of an ambulacral area of Melonites multiftorus. b, Portion of an am- 

 bulacral area of Oligofiorus Dance: z, Lateral row of interambulacral plates. Carboniferous. 

 (Meek and Worthen.) 



the normal ten plates, but the ocular plates are sometimes imperforate, 

 and the genital plates are furnished with from three to five pores. 



Oligopoms (fig. 247, b) is very similar to Melonites, but the ambulacral 

 areas consist each of only four rows of plates. 



Allied to Melonites is the Carboniferous genus Leftidesthes, in which 

 the ambulacra are composed of no less than ten rows of plates, the inter- 

 ambulacra being comparatively narrow and composed of six or seven 

 rows of plates. The plates of the test, as in various other Palechinoids, 

 are imbricated, the test thus becoming flexible, as it is in the recent genus 

 Asthenosoma. The imbrication in the flexible Palechinoids differs from 

 that of the Echinothnridce in the fact that the overlapping of the inter- 

 ambulacral plates is from below upwards, and that of the ambulacral 

 plates from above downwards, the reverse of this taking place, in Astheno- 

 soma and its allies. 



A well-known Carboniferous genus is ArckcEocidaris (fig. 248), in which 



