158 THE DEPTHS 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 pian of the Dia- 
dematide, 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 areze from the apical pole towards the 
mouth, those of the ambulacral areze 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 
ares 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 ares 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 arew; the 
remainder of the surface of the plates is thickly 
studded with secondary tubercles and miliary grains. 
