] 58 THE BEtTHS OF THE SEA. [chap. iv. 



and five large round openings in the ovarial plates 

 in the centre of which open the wide ducts from the 

 ovaries. The jaw pyramid, 'Aristotle's lantern,' is 

 large and strong, and formed on the plan of the Dia- 

 dematidse, and the teeth are large and simply chan- 

 nelled. The point of structure, however, in which 

 Calveria differs from all previously described recent 

 urchins is the arrangement of the ambulacral and 

 interambulacral plates. These, instead of meeting 

 edge to edge and abutting against one another so as 

 to form a continuous rigid shell as in most other 

 echinids, overlap one another ; the plates of the inter- 

 ambulacral arese from the apical pole towards the 

 mouth, those of the ambulacral arese from the mouth 

 towards the apical disk (Fig. 28). In Calveria, the 

 outer portions of the interambulacral plates leave 

 spaces between them which are filled up with mem- 

 brane, and the inner ends of the plates form large wide 

 expansions, which overlap greatly. The ambulacral 

 pairs of pores are singularly arranged : they are in 

 arcs of three, but two of the pairs of each arc penetrate 

 small special accessory plates, while the third pair 

 penetrates the ambulacral plate near the end. The 

 outer ends of the interambulacral plates overlap the 

 outer ends of the ambulacral plates, so that the 

 ambulacral arese are essentially within the interambu- 

 lacral. The interambulacral plates bear each close to 

 the outer end where they overlap the ambulacral 

 plates, a large primary tubercle; and two imperfect 

 rows of primary tubercles bearing long spines are 

 ranged in the middle of the ambulacral arese; the 

 remainder of the surface of the plates is thickly 

 studded with secondary tubercles and miliary grains. 



