162 THE DEPTHS OF THE SEA. (CHAP, Iv. 
to the mouth the distinction between ambulacral 
and interambulacral areze is apparently lost, and 
the sutures between the plates can scarcely be made 
out; the pore ares are reduced to mere lines of 
double pores, and the whole of the surface of the 
shell is studded over uniformly with the very large 
areole: of primary tubercles, bearing spines which 
are small and delicate and apparently quite out of 
proportion to the mass of muscle connected with 
them which fills the areole. As in Calveria, the 
tubercles are perforated. 
We have thus become acquainted with three mem- 
bers of a family of urchins which, while differing 
in a most marked way from all other known living 
eroups, bear a certain relation to some of these, and 
easily fall into their place in urchin classification. 
They are ‘regular echinids,’ and have the normal 
number and arrangement of the principal parts. 
They resemble the Cidaridee in the continuation of 
the lines of ambulacral pores over the scaly membrane 
of the peristome to the mouth, and they approach 
the Diadematidée in their hollow spines, in the form 
of their small pedicellariz, and in the general structure 
of the jaw pyramid. From both of these families they 
differ in the imbricated arrangement of the plates and 
in the structure of the pore aree, to the widest extent 
compatible with belonging to the same sub-order. 
Many years ago Mr. Wickham Flower of Park 
Hill, Croydon, procured a very curious fossil from 
the upper chalk of Higham near Rochester. It con- 
sisted of a number of series of imbricated plates 
radiating from a centre, and while certain sets of these 
plates were perforated with the characteristic double 
